Selena Gomez -- Justin Bieber's Ass-Kissing Won't Work ... This Time

Selena Gomez isn't falling for Justin Bieber's old tricks anymore ... she's exercising her rehab knowledge and slamming the door shut on any future relationship.

Sources close to Selena tell us ... Selena scoffed at Biebs' Instagram photo of her at an Oscars party with the caption, "Most elegant princess in the world." Sorry J.B. ... we're told she laughed out loud when she saw it, saying it was stupid and childish.

Selena's party line ... she went to rehab to get over Justin and it worked. The reality ... she went to rehab for booze and weed. Good luck, Selena.