The former President was permanently banned from ever rejoining the actors' union this weekend after the board passed a resolution that gave him the boot for good. He'd already resigned -- going out swinging too, no less -- but now they've cut ties as well.

With the passage of this, they're preemptively denying any possible future application that Trump may submit to get back in. So yes, he's officially banned, expelled, etc.

As for their reasons ... it's pretty obvious. It's all about his involvement in inciting a violent mob to storm the U.S. Capitol Jan. 6 -- and his opposition to a peaceful transition of power on his way out of office. They also say he's been a jerk to journalists in the union, and can't stick to facts or the truth ... all of which SAG says is antithetical to its mission.

SAG Prez Gabrielle Carteris -- who told us punishment was still very much on the table despite his resignation -- says while cutting him out after the fact is mostly symbolic, it's important ... adding, "It is a resounding statement that threatening or inciting harm against fellow members will not be tolerated. An attack against one is an attack against all."

What this means big picture ... Trump won't be able to do any legit TV or movie appearances in the future, at least none that are SAG sanctioned.