DJ, as the thief / code breaker is known, sports a hat that says "DON'T JOIN", according to his entry in the official Star Wars Databank:
A gifted slicer, the man known only as DJ used his skills to steal from wealthy gamblers passing through Canto Bight. Profoundly cynical, he saw the First Order and the Resistance as halves of the same corrupt money machine. He wore his simple philosophy on a plate adorning his hat: DON’T JOIN.
This is consistent with his statements to Finn and Rose in the film, including his revelation that the First Order and the Resistance buy weapons from the same dealers.
Here is the writing in question (written in Galactic Basic) on a licensed toy of DJ: