I haven't seen a mod to turn it off yet. Hopefully the Nraas mods will soon include this ability (or at least let you set the social group level to 0).
There isn't much to be done about them the modless way. I believe the standing can eventually decay so the sim no longer belongs to that social group, but that's tricky to accomplish (the sim would have to be unable to interact with anyone or anything that would increase social standing, plus be unable to do any self-social activities like the school cheer).
I understand wanting to turn these influences off, trust me! After an immortal dynasty of nerds berating each other I finally had them gain jock levels just to try and heal the constant relationship wounds, only to suffer through seven sims breaking into 'Grrbits, grrbits, go grrbits!' every other minute. The social groups are an interesting idea, but their interactions happen much to frequently for my tastes.