"tor" can be blocked
Alot of work is involded in blocking some one that knows how to use relays/proxies/tor and other such powers.
but it can be done. Alot of proxies send a diffrent HTTP header then a normal
Internet Exlopier/firefox HTTP header.. blocking them http headers would block about 70% of proxies. The hardest part would be blocking the level 5 anonymous proxies.
As they will hide your IP Address 100% also will clone a standard web broswers HTTP Headers so no one can tell unless then have software running on there web server to scan every ip address that connects to there website for open Proxie ports and then trys to connect to see if it really is a proxie.
( all in all. If he knows what his doing he will get on if he don't no nothing you can block him in a heart beat
)