What should I do about this personal vendetta?

So, my girlfriend’s father and I have had a grudge with each other since I started dating his daughter, which was 3 years ago. She and I are both in college now, but he does anything he can to try to get us to break up.

He’s photoshopped pictures of me in with other girls and guys, doing dirty things. His most recent thing is threatening to keep my girlfriend’s younger siblings from seeing her if we don’t break up.
I can’t say that I’m lily white as far as getting back at him goes. I’ve called and left some pretty rude messages to him. Calling seems to not work very well, and I want to move to something that will have a reasonable effect.

He refuses to sit down and talk with me about it, so I guess that’s not an option. If it’s at all possible and legal, I want to fight him.

That’s the thing, though. It has to be legal. If I even get a ticket, I’ll lose my $11,000 scholarship. I come from a poor family, and losing my scholarship isn’t an option.

If it matters at all, he’s got his wife, his ex wife (my girlfriend’s mom) the new wife’s kids, and two of my girlfriend’s siblings in on the Amway pyramid scam. He is at the top of his group, and he’s not informed them that it’s a pyramid scam. I guess that’s irrelevant, but if it can help me bring him down, let me know. Like I said, I really just want to kick his as*.

I’m 18, and he’s 36, if that matters…

Thanks, guys!

Also, I apologize in advance if there are any spelling errors.