I generally let them merge in, even though when traffic is coming down off ramps they are supposed to yield by law, often they are staring straight ahead while blabbing on their cell phones, so I find it easier and safer to just speed up, merge left or slow down, in order to let them merge in. Not worth battling over space even if they are in the wrong. Do not need an accident. I see it almost every day when there are lane restrictions coming up, signs warning about right or left lane closures 2 miles back, but still many ignore it and then race around wanting to be let in at the last 100 meters or so. They are basically being a jerk, but better to let em in than be part of an accident. Around here I pretty much expect some one to come zooming around when a lane restriction is coming up.
I find it easier and less stressful to let em in if I can do so safely, Sometimes have no choice than to block a business driveway due to the light timings , but if I have enough reaction time I will let them in.