Recording Audio for Stand-Up Comedy
It depends on the environment, but generally, you want directional microphones that you can point at the audience from in front of the speakers.  In an auditorium designed to spread sound around though, you are always going to get some of the speaker in the sound.  Directional mics will focus the direction of sound they pick up towards the crowd, but some sound still bounces back.

Another possibility, since crowd noise is generally fairly similar and non-distinct, it is entirely possible that they aren't using the crowd's actual sound.  They may just be using laugh tracks to emulate the audience.  This can get clean sound since there wasn't anything else going on when it was recorded.