Polish slang phrases - most popular.

Onet translated "No more Mr Nice Guy" as "Koniec bycia miłym facetem" which doesn't quite do it.

Is there an equivalent Polish expression?

I don't think there is a very close equivalent in Polish, particularly related to the "Mr Nice Guy." A more general phrase could be used, eg. Koniec tej zabawy.

Yes!

Koniec zabawy fits very well.

Most people on PF will understand that but for those that don't, it means stop playing(Games) as if you are talking to children.

And yes, it does fit very well.

Alternatively; Koniec cackania się, or Koniec dalszego certolenia się.