You cut them off when they display signs of intoxication. Lacking that, no, you have neither the obligation nor the right to refuse service to someone otherwise of age to consume & who
has the means to pay.

As it is with gun sellers. If an individual is legally entitled to make the purchase & shows no indication of doing something illegal with it on what basis does one not complete the sale?

A seller who believes the AR or similar doesn’t belong in civilian hands can always choose not to stock them. Otherwise s/he will face lawsuits & possible criminal action of a different kind.