Find a way to mention your scapula. That's right. Open up a conversation where you talk about your scapula in relation to your training in general and bar placement in particular. The poor bastard won't know what the fuck you're talking about, but he'll shut up. This is how I would get CANFIT Pro PTs to fuck off. Because they think they know what they're talking about. As soon as you use terminology outside of "core" and "balance," they'll think twice to bother you again.