that tears it
1. That's been ruined or spoiled. Well, that tears it for our chances at getting re-elected. A: "That didn't sound good." B: "Yep, that's torn it—the engine is completely shot now."
2. I've had it; that's enough. OK, that tears it, I'm calling the police—it's 2 AM and that party is still blasting music.
that's torn it
That's has ruined, foiled, or spoiled everything. Everything had been going well with the deal for the merger, but after we found out we'd have to fire half our staff—well, that's torn it. That's torn it, then—we can't have the tournament if we don't have a referee.