Reggie Bush -- I'm NOT Back with Kim Kardashian

Reggie Bush ain't going back to the Kardashian well ... telling TMZ he's 100% NOT dating his old flame Kim ... despite an expensive lunch date this weekend.

Reggie was hanging out on Rodeo Drive in Bev Hills when we asked if he was back on with his reality ex. Reggie couldn't have been more clear -- hell no.

TMZ broke the story ... Reg and Kim were spotted on a lunch date at the Beverly Hills Hotel this weekend.