What is the best way to tell a man you're just not interested- without hurting his feelings.

I belong to a popular online dating site and get a lot of responses. The problem is having guys monopolize my time and attention every time I'm online. I have several men that seem to appear every time I log on. I've turned them down for dates and haven't taken them up on their invitation to "chat" via phone or e-mail but they still seem to just not get it. I've tried the, "I'm not interested" button and the response is sometimes downright abusive.
I keep my profile hidden and and rarely log on because of this...but what's the use in belonging to a dating site if I'm uncomfortable using it?! How can I firmly, but politely, tell someone not to contact me again?