An executive-level manager from a non-Support team keeps assigning one of my team members tasks directly without involving me. This work takes up a lot of my time and impacts my team’s performance goals. This executive-level manager is much "higher up" in the company than I am. I have asked him to please include me when such requests are made of one of my team members, but the executive-level manager continues to make direct requests anyway. How should I respond?
If you're the manager, then inform your staff member that they need to get approval from you to action anything from outside the team. This is normal procedure, it makes sure you're in the loop for everything and can schedule work around superiors needs.
It also means that every single time your staff member is interfered with, you're basically reiterating to the superior in question that you need to be in the loop. They're unlikely to interfere with how you run your team by overtly telling your subordinate to do something and ordering them not to tell you. But if they do, then that is a whole other issue.