If they aren't calling you, don't answer them. Don't interrupt a QSO unless you have something to add to the topic at hand. There are few things more frustrating than being in the middle of a meaningful QSO to have someone break in to ask an unrelated question or make a pointless comment. Talking for the sake of talking. Eventually, all QSO's must come to an end.
Good operators know when to call it quits.
There are others probably waiting to use the system, even if they are not interrupting the QSO to make a call. If the QSO has gotten to the point where the parties are struggling to think of something else meaningful to say, it's probably time to say 73. 4. If you receive bad signal reports, either due to a weak signal or poor audio, stop transmitting until the problem is resolved. If the condition is that you are weak into the repeater, the problem might be resolved by you moving to a better location if using a handheld radio, or installing a better antenna, or increasing power. If you receive complaints of distortion, muddy audio, alternator whine, hum, etc., fix the problem, don't ignore it.