I only get annoyed when they don't move out when they have plenty of room to. I.e. they are either being lazy or deliberately trying to "scare" me. I would get a flag but I often want to squeeze through gaps in traffic that are smaller!

Just keep riding and try not to let it phase you. Obviously if they blow past fast / close enough for you to feel the wave of wind push you a little then give them a finger - but always remember they may have a gun / knife / weapon or want to fight.

I'm trying to chill out while I ride as it is a good way to relax unless you get road rage!