Try tea tree oil or something with that in it. If it's really bad, go to a doctor. You don't even have to go to a dermatologist - your general practitioner can prescribe things to you.
Also consider that it might be folliculitis and not acne - in which case acne treatments won't help.
One thing I did last year that really helped my cystic back acne (why I'm not doing it now, I have no idea; I must be an idiot) is to alternate days of washing with BPO and washing with pyrithione zinc. BPO kills acne bacteria; pyrthione zinc kills the fungus that causes folliculitis. So between the two, you're killing both. Pyrithione zinc is the active ingredient in Head & Shoulders, but you can buy stronger pyrithione zinc bars online.
So you wash one day with a pyrithione zinc bar (leaving it on for at least 2 minutes before rinsing), then wash the next day with 10% benzoyl peroxide bar (PanOxyl makes one that you can get in drugstores), and so on, it might work well for you. You can try using head and shoulders first, but I recommend getting the zinc bar on Amazon or something instead, because the shampoo is only 1% and has other crap in it that's meant for your hair; the bar soaps are much stronger and formulated for skin.
Once it's clear, you can maintain it by keeping that up. Or, you can cut back to just the BPO soap and get a BHA or AHA lotion to apply post-shower. Dan's AHA works really well, and so does Paula's Choice weightless body lotion, which is BHA (I talk about it in my acne.org blog if you want to see more). I use that right now. I'm still washing with PanOxyl because I had a breakout last month I'm trying to kill. But try the routine I outlined above.
There's also apparently some kit out there by a dermatologist named Dr. Bailey that contains a bottle of BPO wash and a bottle of zinc wash and some special cloth that she put together for body acne sufferers. I haven't tried it myself, but I hear good things about it and used that alternating days principle she recommended when coming up with my own.