If (and [I think] ONLY IF) punishment is certain does punishment contribute to prevention. The classical example is the person who places his hand on a hot stove. There is no doubt about the consequences, and hence those consequences constitute serious prevention.
Very few episoes of spousal abuse lead to any legal consequences. Nothing Police can do would be preventive of domestic violence.
Very few serial kills are caught through the work of police departments. Almost certainly the rate of such crimes would be unchanged by the abolition of police departments and prisons. Aside from that special case, murderers seldom repeat their crime. Hence punishment of murder is almost always a simple and glaring case of closing the door after the horse is gone.
Without the (false) assumption that the police "serve and protect," men and women in a given community would begin to think seriously (and collectively) of measures to reduce some kinds of violence. Threats to property can be ignored. And of course the war on crime is perhaps itself the worst crime of all.
How do we stop or at least reduce sexual assaults? Certainly the police are not going to help on this, and only the most naïve utopian would suggest "reforming" the police for this purpose. In fact, in general arguments for the usefulness of police are utterly utopian; they are simply mental exercise with no reference to actuality.
P.S. I am no being flippant. I am seriously suggesting that _at best_ police have no effect on crime; at worse, they raise the crime rate.