I've always used face washes on my back and shoulders, in fact i could get away with something stronger as my skin was less sensitive there. There are some goot OTC spot cremes which have stuff like benzly peroxide / other peroxidy things which are pretty good at clearing stubborn spots - but you can't use them as an all over wash or you'll end up red raw. I found oxy stuff stronger than clearasil which in turn seemed to be stronger than clean and clear - but this could just be due to my skin type - there definately seems to be a lot of variation on what works for whom when.
You should try some others - also don't be shy asking your pharmacist/ their assistant - they'll know what's new out/ the strongest thing available where you live (varies between countries). It's been a while since I've worked there so I can't remember what's licensed at the moment
Failing that the doctor could perscribe you something stronger- but try the OTC stuff first.
Good luck