Go gentle, I've not thought a lot or researched this idea, it just popped into my head last night...
Make paying for sex a criminal matter with unlimited fines for johns... UNLESS they are on the "Johns Register". If you want to use a prostitute without being at risk of prosecution you need to be listed on a public register of Johns. Name, town, age, photo and evidence of the results of an STI test conducted in the last 3 months.
One can debate the ethics of using prostitutes, but men who believe it is morally acceptable to do so should at least have the courage of their convictions and be open about it. They can't have it both ways and say it's fine but I have a problem with people knowing.