At least for me, the unforgivable behavior is stealing tools. One guy would go through the joboxes of other trades while they were on break and look for tools to “borrow”. He eventually got caught, roughed up and quite a few of stolen tools recovered. No need to call the cops, as he got fired and the union local was notified, so no work for him for awhile, as the nearby non-union shops were also made aware. Tool stealers eventually get caught.
I was a union iron-worker for 15 years in NYC. On one job a guy I worked with as partner had like 10 children. During that time one of his sons owned a boat engine repair shop. A worker there stole tools so the son took him out on a boat and left him to drown out in the ocean. I don’t recall what happened with the son but Pete had connections and the story was quickly dropped in the press.