The correct hand signal for letting other riders know that they pissed you off, is to... Extend your left arm straight out with your elbow bent 90 degrees. Carefully extend your middle finger to clearly demonstrate your dissatisfaction with the other guy. NOTE: It is not recommended you do this when you are alone.
I like this method of letting other riders know when they have pissed you off, or made you angry. It works almost every single time because they get so angry when you do this to them. It is a classic.