Out-of-universe

Professor Sub Zero was played by Charles J. Kalani Jr, who was knowknown as Professor Tanaka in his wrestling career. Professor was a common title for a judo master, as this thread attests. Although Kalani was a wrestler, he trained under a judo master, or indeed judo professor, for a time.

In-universe

It's probably something similar. The games in The Running Man are similar to professional wrestling – a game with predetermined outcomes which everyone believes to be legitimate. It makes sense that people would have titles ("Professor," "Dyanmo," "Buzzsaw") that don't necessarily have a relation to their day jobs. Indeed, being stalkers in the game probably is their day job.

Alternately, perhaps "Professor" is meant to suggest mastery, just as with real-life judo.