Has anyone ever asked you to stop playing?
More to the point have you been in a session where someone put a hand on your instrument in an attempt to stop you?
In my mind this is definitely a redline. You simply do not physically try to stop another musican from playing by using your hands. The only time(s) I have experienced this is when a married couple in my session are asked for a song & the singer is not happy with what the guitarist is doing. But they are married, I’m not, so all that is between the two of them.