Not something I experience often if at all, but I'd probably have just played the game (muted them of course) and then reported them after the game. Yeah it's probably a waste of time to some but I can just play a champ I enjoy and get in some practice without stressing about winning the game because if they're going to do what they say then no need to worry about it right?
But, mostly I'd just go through with it to be able to report them after because otherwise I don't see how they'd get stopped. I guess you could send in a ticket about them maybe? Not sure.