To be clear, I'm not some kind of pushover. My awareness is greater for when people pull the whole "push the rules, exploit loopholes, try to surprise ME". I press pause and communicate with the player, and explain what I'm going for here, and that I do not tolerate "player vs. GM" relationships.
EDIT: also I'm more clear and obvious during session zero. I'm more discerning about who I play with too; pre-campaign discussions, as well as session 1, give me a clear picture about the chemistry in the room. I haven't suffered those experiences I mentioned in a long time.