What do I do about PCs using Con damage to “nuke” bosses?

So recently my players realized that they can spam poison-based Con damage on my bosses to effectively nuke them down in a couple rounds. This is basically ruining the challenge of my fights, but they seem to love it. I’m not sure how to handle this without basically saying “No, you can’t do that.” Or making the bosses suddenly immune for some reason. Their entire tactics rely on lowering the enemies saves, then spamming Con Damage to lower it more, which allows easier Con Damage spam, until it’s dead by round three.

I don’t know how to handle this. I don’t want to be an ass and just start doing it back in a “Well fine, if you do ima do it too” fashion.

This sucks…