If you have a particular block of numbers that belong to you it will be easy. You could do several different things, strip digits down to the extension number and send them internally that way. Block it with a COR or Interconnect, send them to a mailbox that yells at them. Many options here, just depends on how creative or annoying you want to get.
The route to strip them may be your easiest solution, how many sites are we talking about and are the extensions in blocks based off of the sites?