Got a friend whose sister works in library and always seems to be on duty when one particular wanker (literally) comes in to use the computers.

He accesses pictures or web sites that are not specifically pornographic, but something on them gets his rocks off and he masturbates in full view of everyone in the library.

I told the friend she should tell her sister to just go up to him, and say in a firm drill sergeant tone ...."Not on my watch, Twiddledick. Get out, unless you want to be here when the police arrive."

Mobile phones are disabled in our local library. Can't get service. There should be quiet areas set aside and rigorously protected, but of course this depends on the size and funding for the facility.

A simple smile at the offender and some comment like "I am not having too much success at this, do you know anything about.....?" Takes them away from the phone and puts them in your corner, so to speak. It is non-confrontational and yet distracts them from the activity that is annoying you. Failing that, grab the phone from them, throw it to Spaw on his way to the loo and ask him to flush it.