What should i do with my colleague?

I recently found out about a colleague/employee’s moonlighting (through another colleague). He’s a full time photographer*.

More specifically, that he uses models our company has hired to take his own shoots using our company’s studios. He advertises them in his own portfolio as his own company, with no mention of us. I wouldn’t be surprised if he used our company’s equipment either.

*As part of our employment contract full time staff cannot moonlight. Part time staff on the other hand, can.

Am I thinking about firing him because i’ve warned him about moonlighting before, and offered him a part-time contract. He chose the full-time contract because it paid better.

Am I overreacting or would firing him be justified?