As suggested on the CurseForge page by LastSpectral, I too think it would be a good idea if the only way for a spawners mob cap to be depleted was for the mob to die, rather than despawn. This way, a player couldn't just wait out the mob cap if they didn't want to fight the mobs directly.

While this doesn't eliminate the possibility of clearing the mob cap without directly engaging the mobs or the spawn (e.g. you could flood the room with lava to kill the mobs indirectly), I think that's still a desirable alternative to waiting (and kudos to those inventive players that create alternative solutions to eliminating the spawner).