When may you deliberately interfere with another amateur radio station's communications?

The rules governing amateur radio are very clear: deliberate or intentional interference with any radiocommunication is never acceptable under any circumstances. Even if another station is operating illegally, exhibiting poor etiquette, or transmitting on a frequency you were already using, you are never permitted to intentionally jam or disrupt their signals. Enforcement of the rules is solely the responsibility of the regulatory authority, not individual amateurs. Options suggesting you can interfere during crowded band conditions or in retaliation are completely false. You must always attempt to avoid interference, even if it means changing your own frequency to get away from a disruptive operator.