People who casually "cheat" but don't submit any kind of score nor publicly distribute hacks shouldn't really be punished, but instead they should be just considered more likely to cheat. Now those that submit a single score while using cheats should be permanently banned, if you ban A and he uses his friend B account then ban B, that's how it is, you are not supposed to share your account, and if your friend is a faggot and makes you get banned too then you should think again if he is really your friend at all.
Multi accounts are not really a problem, if you properly apply ban policies you get to a point where whenever you see a somehow new account get really hard scores, you can just ban him, cause it's OBVIOUS that he is a cheater, someone who got banned in the past or someone using a second account which is not allowed. From the moment you do that when some cheater gets banned he is gonna have to get a new account, play low difficulties for a long time, and then slowly start getting better. If some guy comes and with 500 plays is already playing super-insanes you don't need some genius to understand what's going on. So yeah the previous cheater gonna have to fake play about 20000/30000 plays before being able to start doing SOME hard stuff, and we all know that no one is really that patient. I mean you gotta play 20000 times for real, I guess it is possible for peppy to investigate exactly what you have played (and what results you got) through your life on this game...
Really it's not that hard to deal with, new account getting pro scores = instant ban, even if you are not sure why, you can be sure that he is either a cheater or a multi account.