For a one time flare up just let it heals on its own or else get cream from the vet.
If the problem keeps coming back, the acne is possibly a side effect of some other issue. One of my cats was allergic to all sorts of things: wool, pollen, dogs, corn, fish, etc. His allergies manifested as itchy greasy skin and, yes, acne. We didn't have dogs at the time, so owning a cat that was allergic to dogs was merely amusing. We replaced our wool rugs with synthetics and put him on a special diet. I've heard some cats improve when switched away from plastic bowls. We had always used glass bowls, so I never tried that experiment.