These mfs keep trying me.

I've remained calm and unbothered by this but there's days I wake up choosing violence. So, one of my jobs requires me to engage with customers at a store setting I don't work for the store I'm just a vendor and they let the company I work for demo products there for sales purposes anyway. I'm starting to get irritated by certain people there there's a guy who works there and he's always telling me that I look unhappy about how unhappy I look he keeps making these comments when he walks by me I always tell him maybe it's because you're talking to me that's why. Then there's customers usually older white boomers who keep telling me to smile you don't look happy. So any advice what I can say to these people so they leave me alone I'm just there to work and I'm tired of people thinking they can tell me something. I usually take it too far and I will just say something straight up mean like how I tell that guy yeah it's because you're talking to me maybe if you weren't talking to me I would be happy. Any advice?