FA's are trained not to create a confrontation over the issue. The "rule" is not a rule, it is simply a way of saying that he cut you off. For better or for worse, the rule is:

"No certificate holder may serve any alcoholic beverage to any person aboard any of its aircraft who -
(1) Appears to be intoxicated;" 14 CFR 121.575

Note that the rule is not that one is intoxicated only that one appears intoxicated.

You can send a note to UA, but it will back up the FA and the FA will have documented the incident in any event.