Antonio Brown's longtime personal trainer has completely cut ties with the Steelers superstar ... saying working with A.B. is "bad business."

A.B. and Florida-based fitness expert Bo Smith were inseparable for years ... and the duo had documented their intense workouts on social media.

But that's all come to an end ... with Smith sharing news of the messy breakup on Instagram, saying "I had to walk away & remove myself from the bulls**t! Be a MAN of your WORD and own up to your WRONGS as a MAN should, Communication is KEY!"

He added ... "I've learned to walk away from anything or anyone that THREATENS my PEACE of MIND, SELF-RESPECT, VALUES, MORALS or SELF WORTH."

Unclear exactly what caused the rift -- but it doesn't sound good. 

The situation sounds similar to Brown's split with his personal chef -- who split from the NFL star last year in a public, bitter breakup. 

Smith should be able to rebound -- he's trained a laundry list of big athletes including Chad Johnson and Geno Smith