MTV has instituted a zero tolerance policy for  "Jersey Shore" runaways -- after nearly losing both The Situation AND Vinny -- threatening to fire any cast member who even thinks of going AWOL.



Sources connected with production tell TMZ, the network is serious this time: if another cast member tries to walk off the show ... that person will be fired, and the remainder of his or her contract will be terminated. No exceptions.

We're told it's all a power play. With two cast members nearly jumping ship in the same week, MTV wants to reassert some authority ... and show the cast who's boss once and for all.

But it might already be too late.