I like to catch these guys out - I always signup for services using "servicename@mydomain.com". For example, I signed upto the popular newsgroup service "alibis" and used "alibis@mydomain.com" (domain mydomain.com for example only)
Since all emails @mydomain.com come to me, i can selectively block them one they're outta hand.
Anyway two months later I start getting all this spam to that account. Bingo, guess who's just sold there lists
Robert.