Fresh Prince Star Slams Will Smith

15 years after the show went off the air ... Janet Hubert tells TMZ she's still upset with the way she was fired from the show ... and blames the whole thing on Smith himself.

"There will never be a reunion ... as I will never do anything with an a**hole like Will Smith," Hubert said.

She adds, "He is still an egomaniac and has not grown up. This constant reunion thing will never ever happen in my lifetime unless there is an apology, which he doesn't know the word."

Hubert has said in the past she believes Smith is to blame for her departure in the middle of the show's run -- claiming he turned on her because she wouldn't kiss his butt.

But Smith refuted the claims after Hubert left the show in 1993 -- telling an Atlanta radio station Hubert simply had a bad attitude which played a major role in her demise.

Smith told the radio station, "[Hubert] said once, 'I've been in the business for 10 years and this snotty-nosed punk comes along and gets a show.' No matter what, to her I'm just the AntiChrist."