Can "But!" be a sentence?

This is an emphatic interjection used to indicate dialogical flow and break. In the natural flow of discourse a brief interjection may be used as a sentence-like enunciation. It can help reproduce or translate dramatic pauses or silences.

At any rate, such expressive use of punctuation and fragmentation is generally not acceptable grammatical composition in formal writing.

It's what someone (fictionally) said in a tense situation: it isn't required to be 'grammatically correct', anymore that someone would worry about being 'grammatically correct' when shouting at someone in an argument.

But! No! You wouldn't usually write like that in a formal document.