Here, read this...
http://www.cnn.com/2005/TECH/space/0...eut/index.html
So you get a small black hole and take a mass of something, you don't really need a star, at this speed just a big rock will do. Get it orbiting the black hole real fast and then at the right point give it a shove so it shoots off in the direction of whatever it is you want to smash really hard.
I've heard that some physicists have speculated about the presence of micro-sized black holes. If you could capture one of those and use it to sling some projectiles I'm thinking you could put a decent sized hole in someone's ship.