Not in all the cases no, Demons do sometimes use their brains you know. I did once get a Hunter, got a guy roped in on a deal to do me a favour for some extra time on it, he seduced her, and when she wasn't paying attention burned through her tattoo. Nothings foolproof.
It is.
Exorcise him. You'll likely have to put him down first, and unkillable or not, there are 5 things the Colt can't kill that I know of and do you really wanna put money on finding out if you guys are 6 on that list