Apparently you don't understand your job description.
You're a bartender - which means you fill drink orders from guests.
You're not a dictator, just because your establishment has a cocktail list
doesn't mean that your guests are restricted to only ordering beverages
included on it.
Where did you get this perception from? It's ridiculous. Your bar should be
fully stocked with a number of different brands of liquor and a variety of
liqueurs. The only instances in which you should be denying a request from a
guest would be if they are too intoxicated, you simply don't have the
ingredients, or the request is illegal (i.e. exceeds the allowed serving size
of alcohol per beverage).
Maybe you're just frustrated because your drink knowledge is limited, or maybe
it's because your service skills are missing in action. Regardless, the
bartender who requested the drink probably felt the need to tell you he was
bartender also because he was dumbfounded by your attitude and the look of
confusion that was probably plastered across your face.
Either way you should change your attitude now, because it's not going to get
you very far.