"No" needs to be said before the roll, IMO. Then If the player insists on doing something impossible anyway, just role-play the failure. With that said, actions that are in a narrow sense impossible can still have positive outcomes and if there's the potential for that then I'd say roll for it. The proverbial dragon seduction attempt can still possibly flatter a dragon with a big ego enough to benefit the PC even if it doesn't get the PC laid.