Blocking numbers?

Is there a way to block a specific number? My girlfriends sister has this guy that keeps calling and txting her and was wondering if there is a way to stop that? Can I go to the store and get it blocked or what do I need to do?

What kind of phone?
Some smartphones have apps that can be downloaded. I have a Samsung Blackjack running Windows Mobile 6.1 Standard, and use an app called "CallBlock" that works great.

If you have a Nokia, there are great apps out there. Any other non Smartphone, the easiest way to avoid non wanted calls, just change the phone number. USCC doesn't block numbers on their network without phone based applications.