I used to have a "Red Card", like football referees do. And everytime I caught somebody speaking Spanish I would give them the red card. The one who had it by the end of the class or the most times had to present a 5 minute oral lesson the following day on a topic I selected.
It improved behavior, since none wanted the extra HW and got them all to speaking English at once
Something that I found very useful too is to stick to the rules for quite some time, cause sometinmeswe feel so overwhelmed that we change the rules every class and they dont get used to them.
Providing some time every week to talk about whats goign on with/among them (even in their native language) is also useful since it releases pressure
Last, but not least, NEVER show that you "dont know what to do" cause they �ll see thru and make your life MISERABLE
Good Luck with them