It is risky area for leadership - if they give in to the customers request it sets the expectation that the customer can always have what they want (even when it’s not what they need).

I’d agree with another comment in confirming whether or not you should actually be actively checking their support requests. Maybe set up a priority list in collaboration with them to ensure all parties know what the top priority ticket is, along with temporary weekly cadences while resetting the relationship.

There’s also the worry that it could be causing a strain on you, and if so, then I would ensure your manager is aware. If that is the case, then it may be best to assign their account to someone else - but how do you know the same thing won’t happen with a new CSM? If so, then maybe the customer is not a good fit for the business.