Sometimes there are newbies who legitamately didn't merge earlier if they are unfamiliar with the area. However you always know the idiots who do it regularly, you can just tell by their movements. Fortunately I drive a older large truck with a metal bumper and they usually drive small BMWs or Toyotas with very thin and scratchable side panels. However, most of the time the douchebags do win b/c I don't want to deal with accident reports. However there have been times when I'm fed up with it and either don't let them in, or get within an inch or two of their side door.
On the other hand, it isn't worth some crazy asshole getting out of his car and shooting you in the face for not letting him merge, so usually I just chill and let it go, it's just not worth it.