I've always used "Hibiclens". It's a surgical cleansing soap. Must be used careful and not go into the mouth, nose or ears at all.
I get a few guazes, barely wet them, pour on a little Hibiclens (it's pink), scrub, scrub, scrub the chin and around the chin, letting nothing go into the mouth (keep a dry, clean one to wipe any foam that may get to near. Get another one and do the same thing - this will get a lot of the feline acne off. Then rinse, rinse and reeeeeally rinse until ALL pink is completely gone... and then dryyyyyyyyyyyyyyyy dryyyyyyyyyyyyyy dryyyyyyyyyyyyy so there's no wetness left to create any bacteria for the feline acne to grow anymore.
Using ceramic or metal/stainless steel bowls are best. Plastic grows bacteria.
Using a fountain for their water is a great thing for them as well as they often drink straight from the "waterfall", not getting any moisture on their chin (thank GOD I have one of these for my Himmy!!).
Anyways, that's about it on feline acne from this end.
Warmly,
Julie O'