I have one now that sends me messages on Yahoo Messenger. I want to find out how I can stop him from harrassing me dailey. He just doesn't get it that I'm not interested in him. We met and it didn't work so I want this insanity to stop.

There is a similar disorder predominately afflicting males called Narcisitic Personality Disorder or NPD. If you do a search you will find many support sites with a wealth of information detailing characteristics of NPD's (and others) . There is no successful treatment for the disorder to date.
The advice given is NO CONTACT whatsoever with the NPD. None! The sites give advice on how to distance yourself from the NPD.
There is also info on BPD and other disorders on the sites.
Good Luck.