I just pretend they aren't there. If there's room, I don't care if they "cut in". That's just pragmatic New-York-style driving. However, if they cut me off, I don't slow down or change my trajectory. Braking just teaches them that you'll accomodate their rudeness, and braking also causes those backward-propagating traffic waves.
I haven't hit any of them yet, but I think a few of them were startled enough to learn something.
cross-posted from another thread