I'm suffering from some of that dreaded, my wonderful first grade ss's have turned into lazy uninterested second graders -itis.  Is that a thing?  Oh well. In my school this attitude change comes , in part, with the students finally being corralled into their educational direction during the second year of study.  They have stopped being easy going 1st graders because now in their second year they are "serious" students either studying in the "science" track of education or the "social science" track.  It is a pretty common thing from my understanding for the students to become more serious about their specialization as high school moves on.   It is largely my "science" tracked classes that have become the most burdensome to teach.  This to me makes sense since students who are generally motivated in learning foreign languages tend to be the ones who are interested in the "social science" track anyhow.
This leaves the question as to what the hell do I do with my ss's in these science track classes since they seemingly don't want to do English.  Well if you have a Korean co teacher in your classroom I'd suggest using him or her to generally keep the kids in order.  I don't have one.  I rely mostly on my students at least having a little respect for me to keep the noise level  where I'm not constantly screaming over 20+ ss's.
This doesn't always work but since I've pretty much been told I can't single out students for discipline well what can I do if they decide they don't have that much respect for me.  (Answer,  have a talk with the homeroom teacher about specific ss's and the problems they are causing.)
I think the trickiest part for me is learning to ignore the "bad apples" and teach to the students who still are willing (some of them even want) to do the work.  It is easy to ignore the good students.  They tend to be the quiet ones.   I think the key is to focus your lessons on the students you can work with and try to make it such a good time for them that slowly more students want to join into your activities.  It seems like an incredible task but it seems to be the only real option I have. 
Regarding candy. I've used candy as a prize but it seems to only really work when I have my extra curricular classes where I have far fewer students.  These students also joined this class voluntarily.  Candy might work for younger ss's but high school ss's have largely never seemed to respond to candy to sweeten the deal.  (see what I did there.) 
Anyhow don't know if I offered you any helpful insights but I do offer my sympathies.

I'm glad I'm not the only one going through this.  I teach three second grade classes, at an all boys school, one day a week.  I'm alone in the class and getting them to just stop talking is a massive task.  I threw a kid out, today; it felt good.  Doing so seemed to make the rest of them behave a bit more.  Kid wasn't even the worst one; he just happened to be the loudest at the wrong time.  I've tried moving the odd student to desks that are isolated from their friends, but that seemed to have little effect on them.

On the flip side, my first graders are a decent bunch, which is a massive improvement to when I had them in Middle school.

I can relate to your issue for sure!

As for me it's my Grade 1's who are a bit of a problem. I think it's because I have no discipline system set up for them.

What I do with my Grade 2's (and it works WONDERS!) is this:

-Bought a large world map and posted it up at the back of the classroom.
-Every class I spell "ENG" on the white board at the front of the class.
-If the students behave well, I add letters and eventually spell, "ENGLISH" on the board
-If the students misbehave, I take away letters
-Once all the letters are gone, I take away minutes and they have to stay after class

-If the students spell "ENGLISH" by the end of the class, they choose a country on the world map as a class and I put a little sticky on that country.
-I tell the class that they are competing with ALL of the other classes (I have 15 grade 2 classes)
-The winning class with the most countries/stickies on the map gets a special prize at the end of the semester (last semester it was krispy kreme donuts, which you can buy a lot for really cheap)

It's gotten to the point where I just say, "3, 2, 1" and they become quiet right away. If they aren't quiet I instantly take away letters from "ENG." Then some members in the class start yelling to others, "QUIET!"

I know you said you don't give away candy, and with that I agree (after all, they are almost 18 and 19). But I think that because their class isn't graded at my school some of them have no real motivation to study, do homework, etc. So I had to implement this system in order to keep them well behaved and more interested. They LOVE choosing countries and even saying the names of countries.

I hope this helps! It helped me out a lot, and my classes run much more smoothly because of it.

These are all really good ideas.  Thanks for your input, everyone.

I'll share one motivation tactic that I have with my science track classes:

The only book I use with them is Word Pot.  They have 20 words for each week that they have to memorize.  We listen, repeat, three students read the words once, I explain the meanings in English, and give them some sort of vocab test.  This is as boring as it sounds.  I struggled with this last fall so I decided to make class more fun.  I implemented a system I call Ice Cream Fridays.  In each class (there are six total) the students who volunteer to read the vocab words write their name on a piece of paper and it goes into a box.  At the end of the week I have a student choose two names and take the winners to the school snack bar and buy them the ice cream of their choice.  It's gotten wicked popular over the weeks and I have no problems getting volunteers anymore.  I also use it if a student has excellent behavior or reminds me to erase the words before the test.  (I value honesty.)  When you take students to the snack bar and a hoard of boys sees their teacher buying them ice cream they get competitive and it builds anticipation.  I'm sure this idea can be adapted anyway you like, but it's worked wonders with my school.  It's not like I have to carry a pocket of candy around, but it rewards those who try.

Wow it is threads like this that make me love Waygook. I couldn't for the life of me understand what the hell happened to my second graders this year, they are rude unmotivated and do not care. This thread has helped me at least make sense of what is going on. I thought they were just sick of me and my lessons, well probably a bit of that too. And we really have remember esp this time of year, our students are STRESSED! Imagine being a Korean high school student and having to come to English class once a week when you want to be studying for midterms. I would probably goof off too.

Best we can do is listen to our kids do our best and try to connect to the ones who care about our classes.