Hm, I can’t really imagine any of the members initiating a fight at all, even for their girl. They’d each be able to handle it maturely and without violence, but the ones that might find it hard to just ignore the guy and walk away would probably be Sunggyu, Sungyeol, and… I want to say Woohyun?
As for the others, they’d keep it simple. They’d tell the guy that she was his girlfriend and they’d prefer if he left her alone. For Gyu/Yeol/Hyun, they’d probably confront the guy and ask him what he was doing, even if it was obvious. Again, I doubt any of them would resort to physical violence. I believe they have the self-control not to do that.