Is there a way to prevent (not relieve) the hotspot caused by wearing a headset for long periods of time?
The best way to prevent the "hot spot" pressure point from the headset band is to wear a headset that doesn't have a top band (like this or this). Of course this requires that you be able to wear an in-ear headset, and some people are more bothered by that than by the headband.

Short of wearing in-ear headsets there are numerous different headset band pads you can try - I know at least one person flying with this sheepskin pad.
Some combination of the right headband position and the right pad should make your headset more comfortable for long flights.