Is it possible to block a phone number from a cell phone?

I’m having problems with an ex-boyfriend calling me. We broke up quite a while ago, but every few months or so he calls me out of the blue. Without going into the whole story, I don’t want him calling me! Ever!
So, can I somehow block his number (several of the numbers he uses) from my cell phone.
If it makes a difference, I have a T-Mobile G1.
Any help would be appreciated.