We are aware of a bug that is preventing the deletion of assets that are also part of escalation plans.
Our development team is working hard to rectify this issue and improve the user experience around managing people. This has ramifications in other areas of our solution and unfortunately is not an easy fix.
In the meantime, rather than trying to delete the user, please simply "Disable" them. This will remove them from the billing database while also removing their permissions.
We appreciate your patience while we work through this and we will be sure to let you know when the work is complete.