Someone tries to pick a fight with you, but doesn't know you are an expert in a martial art.

I blow them a kiss and toss glitter in thier eye...then run!

Standard training in every martial art that has evolved out of Buddhism:
- do not allow oneself to be provoked - remain calm and fully aware
- quietly shift into a stance that enables all possible responses
(shoulders  and hips side-on to the potential assailant, feet wide apart, knees slightly bent, no clothing or baggage to obstruct, arms and hands hang loose and free),
- remain passive until the other initiates a physical attack.

Some versions are more passive than others. Examples are Tai Chi and Aikido;
they take the direction and energy of the attack of the other and redirect or deflect it.

Higher levels of martial arts - wu shu - teach how to observe the potential for violence and avoid it.

Talk is just noise. Threats should raise your attention, but nothing happens until a move is made toward you. Defend yourself, that is what the training teaches. 
For me, my first defensive move would seek to quickly discourage any further assault. Making the would-be assailant look at me from flat on their back is usually sufficient. A second attempt would merit a stronger response, a harder landing, for instance.
The response should be measured to the aggression. But don’t bring your fists to a gunfight.

Subdue the attack but give them the choice to withdraw. Having the skill to maim or kill is not carte blanche to do it. The street is not the Octogon.

Actually martial arts is supposed to keep you out of a fight. Not turn to it as a first resort. If the arsehole won't listen to reason, then you kick his arse. This post was edited by Zack at December 10, 2019 9:35 AM MST

I would paint a picture of the Sheriff of Dodge City ......:)D   

Well, an "expert" would have some sort of video evidence. Footage from sparring or a competitive fight, perhaps. Offer to show it to the person. It does happen quite often...people picking on the wrong guy to mess with.