instance stuck termination

This is a community drive resource site. If you need official AWS support, you should use the Support portal while logged into your account.

That said, Amazon has a troubleshooting doc for this: Instance Stuck Terminating

Essentially it says there are several potential reasons for an instance to take a long time in the "Shutting Down" state, including Underlying host issues, scripts running on the host, and others. They do clean up "stuck" hosts that are in a "Shutting Down" state for numerous hours, though:

If your instance remains in the shutting-down state for several hours, Amazon EC2 treats it as a stuck instance and forcibly terminates it.

I would say if the resource terminating is time sensitive, open a support case, otherwise, give it a couple of hours to be cleaned up manually.