I think if a monster is in hit-recov mode or stunned, they can't teleport. They also can't teleport if they die first.

When my bear druid was fighting the council, one of them was physical immune, fanaticism-enchanted, magic-resistant and fire-immune. Combined with his self-heal skill, this would have been very tough, but my grizzly and I just cornered him and kept hitting him so he never got a chance to self-heal or even summon a hydra :) .