That's that!

(and) that's that

That is final; the issue is settled or completed. You are going to your sister's recital and that's that! No more complaining. That's that! I just send the proofs to the printer. Let's just say that's that and be done with it.
See also: that

That's that!

Inf. That is the end of that! Nothing more can be done. Tom: Well, that's that! I can do no more. Sally: That's the way it goes. Doctor (finishing an operation): That's that! Would you close for me, Sue? Sue: Nice job, Doctor. Yes, I'll close.