Enforcing a hags contract

Most here know what hags are and their dealings so I won't go over that in detail. My question is what can / will a hag do if a deal is broken? "Make their life hell" is a common theme but I don't know if that translates well to the players. Theres always combat but that doesn't seem as fun. Hags are creative and twisted. What could translate well in game for a broken contract?

The idea of curses are fun, but how to handle them mechanically? From my understanding a deal with the hag lacks the universal power of something like a Devils contract. So whatever the outcome the hag needs to handle it herself (or find minions to do it)

I'm looking for ideas that work on players. Npc are easier to deal with cause they can act how I need them to. Something as simple as sending a minion to cause chaos in their home or preventing them from sleeping.

Night Hags have their Nightmare Haunting ability which I have used before. Granting the player exaustion, and reducing max hp. I may have been a little too agressive with doing it every night to the same player. This was in Curse of Strahd, when they made a deal with a hag to get a child back for a pending favor. That favor turned out to be to deliver her 2 children (which I added) from Valaki. Specially the Barons children. When she spoke to them about this they outright refused and the hag simply told them they'll regret it. This ended with the cursed player sitting out for the session as they stayed behind when the rest of the group confronted, and killed the hag. It wasn't fun for anyone I don't think.

So my questions. What can a hag do that's fun and engaging to players? What kinds of deals have you made? What were you prepared to do if the players didn't uphold their end - did it come to it, and was it fun?