Prevent them from being able to clear Alien Nests after getting the level in Purity that gives them immunity from aliens. It's stupid that you're able to walk around and do that in my opinion. If you want to take out the nests, you should be fightin gthe aliens, not ninja-ing the nests by just walking around and hoping the aliens move out of the way.

I would just make it so that Explorers can't enter the tile that the nest is on or something along those lines.

Yes? No?