Find yourself an old hammer. Wipe all the prints off it. Next time he shows up, go down and let him inside. (You'll have to say your girlfriend was trying to leave to go get something at the store, which explains how he forced his way inside. You heard her screams, so you came running downstairs with an aluminum bat.)
Now beat him to a bloody pulp with the bat. Set the hammer beside his body before the police show up. (Put it in his hand first to get some prints on it.) Problem solved.
As long as they have a weapon and have forced their way inside your home you can go nuts on them.
edit: this post is for entertainment purposes only and I don't condone doing anything I have said in this post. ;)