Instant message type system?

[soundbarrierpro]

I have somebody who is literally on my site every single day 8 hours a day going through every single page, link, etc. They don't contribute anything and it appears they're leeching. I've sent PM's and I don't know if he knows how to use PM or if he's just ignoring them.

I'd like to know if there is any type of instant message mod which would pop up on their screen sort of like a network server message.

Thank you