This heavily depends on who they are, and what your relationship is with them, but even if they’re not very close or significant to you I’d say without warning might be a bit too quick. I think you should explain it clearly to them once, then give a shorter reminder the 2nd time, the 3rd time it’s done. Yes, people shouldn’t just be given up on, but there is a point where we need to understand that we can use our time and emotions in a more useful manner.