Another, more standard, symbol to end a proof would be the contradiction symbol. Used in proofs of method reductio ad absurdum ('proof by contradiction'), the Contradiction symbol indicates that an obvious contradiction has been reached and so (generally) the proof is ended. The symbol resembles the Scottish flag with four bullet-holes through it, and is not available in standard LaTeX fonts.

(Brackets added for clarity)

The symbol can be described as ((two parallel lines) intersected by (two parallel lines)) with ((four filled circles) at the (centrepoint of the square which the approximate endpoints of the four lines will form)).