Is it possible to block Tor users?

Blocking Tor is wrong because (ab)users and IP addresses are not the same. By blocking Tor you will also block legitimate users and harmless restricted Tor exit nodes configured with conservative exit policies.

For example, if you concerned about attacks on SSH (port 22) then blocking only Tor will do little to increase security. What you really might need is dynamic synchronised blacklist like http://denyhosts.sourceforge.net/ that track offenders disregarding of their affiliation with Tor.

Denyhosts will automatically block Tor exit nodes that allow Tor to access port 22 without unnecessary denying access to anonymous users and operators of Tor exit nodes who never let offenders to attack your SSH services.

Tor is much easier to block than other open proxies since the list of exit IP addresses is known and published. Read the answer at https://www.torproject.org/docs/faq-abuse.html.en#Bans and if you still want to block users from accessing your site you could use https://www.torproject.org/projects/tordnsel.html.en or the Bulk Exit List exporting tool.

If you use the Bulk Exit List exporting tool be sure to get a fresh list often and expire the old blocks since the list of IP addresses change.