It's funny with those sorts of people. There's really not much you can do apart from making sure you protest after the race. If you're quick enough to pull a gap after you pass them you can go for it, but you have to be about a second a lap faster to be able to do that nicely.
Otherwise, the best tactic I've found is just to stick on their rear bumper and not try and actually complete a move. You can fake moves on them, but you don't want to get alongside them or into a place where they can actually crash you. You just want to pressure them into getting out of their comfort zone so that hopefully they'll lose control and go off track. Wait for them to make a big mistake that will cost them several seconds, then you can capitalise and pull the gap you need to get clear.
Sometimes you'll be stuck behind them all race, but long term you'll get better results by waiting for those people to take themselves out. The reason they drive dirty is that they're trying for results that are above their skill level, and chances are high that they can't hold it together for race distance.
Otherwise, if you work on finishing in the top half of every race you're in you'll quickly get an iRating above about 1600 and you'll see very few of those sorts of people anyway.
