Justin Bieber & Paris Hilton SAY'S WE'RE NOT BANGING

Justin Bieber and Paris Hilton are NOT banging ... in fact, she couldn't stand to spend more than a half-hour with him because he wouldn't stop playing his own songs

Paris and Justin partied in Cannes Sunday night at Gotha Nightclub. There are reports that the 2 got cozy ... with the 33-year-old climbing into Justin's 20-year-old lap ... then leaving for his house.

But here's the deal. We're told 50 people went back to Justin's house -- Paris was never alone with him. And something else we were told ... she said the party was lame and couldn't stand the fact that Justin was playing his own songs ... so she left after half an hour.

So we're guessing "Boyfriend" didn't go over so well.