I will assume your car has a steering wheel. Go around someone if they are driving too slow for your taste. You, too, have the option of switching lanes. No one on the road is obligated to drive any faster than the posted speed limit. In case you forgot, it is illegal. As is tailgating.
Tailgating and light flashing are dangerous, not only to you and the other driver, but to everyone else on the road when you cause a pile-up because you are following too close because you are angry that someone is doing the speed limit.
The left hand lane is for passing, you need to get out of it unless your are passing someone. I repeat, the left hand lane IS for going around someone, it isn't yours to do with as you please