Please remember that if you are touched innapropriately it is classed as assault, if you are touch on genitals, even through clothing, it is classed as indecent assualt, which is a sexual offence.
It has only ever happened to me once when I was younger, by a local councillor of all people, it was about 10 minutes into the treatment, and I stopped the treatment and told him I was calling the police. He swiftly paid my hourly rate and left!
Unfortunately its not something that you can forsee when booking clients, but you can usually see signs when they are on the couch.
I think sometimes a lot depends on where you advertise, local newspapers usually attract unsavoury clients.