'Space Jam' Star -- Sequel Should NEVER HAPPEN ... 'Cause LeBron Ain't Michael

Anyone hoping for a "Space Jam" sequel (all four of you) is about to be seriously bummed ... 'cause one of the movie's stars is telling TMZ Sports why it should NEVER HAPPEN, and it's all about LeBron James.

Thom Barry -- who played MJ's dad in the movie -- was out in Beverly Hills recently when we asked about rumors that LeBron might pick up where His Airness left off in a new Jam.

If you're a fan of King James, prepare to be pissed at this vid -- Barry throws a couple shots at LeBron (and Kobe too) for even thinking they could slide into Jordan's Jordans on the big screen.