"One does not play with bears."
Translation:Man leker ikke med bjørner.
"Spiller" is used in relation to games and musical instruments, so it's a less likely candidate here.
However, should you ever feel the need to warn somebody not to play chess or in a brass band with bears, then it would indeed be the correct word to use.
That's the definite singular form ("the bear"). Here, you want the indefinite plural, "bjørner".
Whats the difference between using "med bjornen" and "for bjornen"? Do they have any specific difference?