The best way to avoid this sort of thing is to kill people with kindness, even if they don’t deserve it.
Say that you’re in the midst of a phone call with a prospect (after some e-mail exchanges), and you realize that he or she just isn’t a match.
(On How About We, this is the “I’m Intrigued” button, on OKCupid it’s a wink.) In general, this is not a good way for a guy to approach a girl on an online dating site.
It says “Hey, I might be sort of interested in you, but I’m going to let you do all the legwork.” Don’t give her a nudge and expect her to then message you, because she probably won’t.
Reporting abuse to the site is far more effective than just blocking a person’s messages (a feature offered on most sites).