Very few of the people who try to get on Rush's show get through. They must reach on open line (not a busy signal), they must get through the call screener (and a liberal-but-learning friend of mine said that Bo Snerdly is real tough, he was always honest about his topic but he never got through, I don't get the feeling that my friend felt rejected because he was a liberal, either). After they get through the call screener they must want on the line for anywhere from 20 to 40 minutes or more (open line Fridays may be more of an exception to this although the lines probably stack up there too).
A troll has to be real dedicated to call Rush's show and put up with all of that. The troll would also have to listen to Rush's show for an hour or so while trying to get through/waiting online. Real dedication.
Even with all of that Rush will let liberals hang themselves with their own rope and go on with their crackpot theories. He may mute them down so he can counter their claims but he doesn't generally hang up on them.
If a caller does try to make a quick insult at Rush, the 7 second delay on the radio allows it to be dumped before it can be heard. Rush will comment that the person showed their true stripes and move on.
Rush is more of a class act than Michael Savage but Rush also employs excellent call screeners who have to stay on the button. This was part of MS-NBC's problem. Caller ID should make it even easier to detect repeat callers.