Have you tried using an anti-bacterial body wash? I use Dial Antibacterial Moisturizing Bodywash and it works well for me (it comes in lots of different scents too) -- although I have never had very bad body acne. BUT, when I met my boyfriend he had really bad acne on his back (and had previously been on accutane), and he started using my bodywash, and things *really* cleared up. I can tell when he doesn't use it because those zits start to reappear. Don't let the "moisturizing" part scare you... it basically just means it won't dry your skin out like some soaps can... there is no oil in it or anything, and it does kill acne causing bacteria...
Another thing to try is Neutrogena Acne Scrub (I used to use it in high school -- I like the scrub better than the wash, for this brand) -- that has salacylic acid in it to fight the acne (honestly, I think the Dial does a better job).
I don't know if you are using either of these washes already, I just thought I would recommend them. I doubt it would cause any problems if you tried a face spot-treatment or something, because your face is probably a lot more sensitive than your butt... if you decide to do that, I would probably put the treatments on at night, and then not wear underwear to bed, and try to sleep on your stomach...
If the zits are really bad and nothing seems to work, you may want to talk to a doctor and get antibiotics.
good luck!
Mirabelle