Although I haven't tried myself, in my experience, every time I have been at the desk at DXB and there is someone next to me trying to bring a guest on a different flight, they get denied, except for when an iO did it, but that's a different case. I don't really want to say strictly, because I do know there are odd occasions where it has happened (at outstations mainly) but staff will deny it has. And if you push it, you will definitely get denied! So. Strict. Yes.

Edit: sample size for this year is 5 off the top of my head, twice at DXB, three at outstations.