If someone is camping in the left lane, I will high beam them if they are driving at an inappropriate speed and blocking traffic. I won't tailgate though; on the highway I feel that is too dangerous. Flashing my high beams is a clear indication of my intent, I don't need to do anything more.

And just to get this out there (I'm sure this applies to no one here): THE LEFT LANE IS NOT THE FAST LANE. IT IS THE PASSING LANE!!!! I don't care if you're doing 75mph and the road is wide open. Other people have the right to go faster, so don't be an asshole and block people's lives.

Once on I-85 south of Richmond, a trailer and 2 SUVs decide to do that stupid "parallel 70mph" thing where they block up both lanes of traffic travelling the same speed. And this went on for 10 miles, and there were 6 other people behind me clearly annoyed. I was flashing my high beams and waiting in the left lane. So when finally a gap opened up, I put my foot down, shifted right, honked my horn as I passed, and went on to 105mph before letting off. (And setting the cruise at a neat 150 km/h)