"Trashtalking" can mean different things to different people.
General bad-mouthing (along the "selfish bitch" line) is not tolerated at all - warnings will go out, and actions taken if they are not heeded.
Criticisms of someone's behaviour will often make me insist that it is said to their face, in my presence if required. Don't say something to me that you wouldn't and haven't said to the person who you are talking about. Stuff like that doesn't go on behind other's backs. I was in that place at one point, and the relationship ended. Won't go there again.
"Listen, or your tongue will make you deaf." - Native American Proverb