Trucks pull up on me all the time. I slow down. One time, one of them decided to bump me. We weren't going more than 15 MPH, still a stupid move. I stopped in front of him and blocked his path until the local police showed up.
He said "prove it". I pointed to the spray paint x-fer on his nice chrome bumper. (I love flat black spray paint, by the way.)
He was arrested on the spot. Dummy.
Anyway - Normally, when ANYONE gets behind me, I tap the brake 3 times to signal that they're too close. Most of the time, they back off. If they don't, I slow down to a more appropriate speed for their following distance. If they still get closer, I just hit the brakes.
Frankly, if they're THAT close, at such a low speed (usually less than 20mph at this point), it's not going to hurt anyone, and I could use a new car anyway.
I've never been hit as a result of this, but I've had MANY single-finger salutes. Almost got into a fight or two at my destination when people have decided to follow me afterward as well. That said - I don't recommend ANYONE do the things that I do. They're probably not the best way to work things out, but they happen to work for me.