How about a clay activity. Really let them have fun, but make
sure they follow your guidelines (including behavior). It's a
real motivator when they lose something they KNOW is fun. Then
you can hold it over their heads all year ("if you want to do
clay again..."). Also, the ones who are causing trouble can be
removed from the project and do a research paper on the history
of (whatever, related to clay, like Greek pottery or something).
I had a 6th grade like this last year. I nearly quit,
actually. But I knew it would soon pass, and since 6th is my
highest level, I will NEVER have to see those rude boys again!!