[Moved from FAQ page]
This is probably because your IP address has been added to our black list. This list gathers all IP addresses hosting harmful activities at least once in the past (server attack,...). If you have just received this IP, the last owner did probably such kind of annoying stuffs or was not securing correctly his machine. Please use a decent operating system and get rid of evil Trojans, spy ware and other mal wares.
The current ban list is available at http://www.geopsy.org/blacklist.html