Hasn't really happened to me yet, but I already know well how leaning or rubbing against the car can swirl the finish, I've accidently done it working on the car (luckily fairly easily buffed out).
Most likely I'd evil stare first, then if they persist, call them out. I had a coworker write "CALIFORNIA DUSTER" on my hood once when my Z was dusty. Needless to say, I got a free hood detail and pretty much full access to a garage/tools
.