Lol I've been called "unprofessional" by no less than 3 employers who tried to screw me on paychecks after saying similar things to them. I work as a freelancer. They ended paying up btw.

"Fuck you pay me" actually works. Their bourgeois sensibilities have zero idea how to handle anything like this. The go-to response of "unprofessional" is hilarious, considering they are the ones not upholding the deal.

Don't scare them though, or they'll call the cops ofc