It should be possible to define anti-spawn regions in a level, like how monsters will not generate in the Sokoban hole corridors but more general. It should still be possible for monsters to appear in specific places within such regions that are specified in the level definition, but monsters specified to appear in random places won’t, nor will any monster that generates after the level is created. This could be done by checking the anti-spawn regions in the makemon_rnd_goodpos function.