FileMaker Cloud instances are created with instance protection turned on by default. You need to go to your EC2 dashboard, click on Instances, click on your running instance that you want to terminate, and then choose Instance Settings->Change Termination Protection from the Actions menu.
Once you do this, you should be able to terminate the instance and/or delete the stack.
Thank you for your prompt response. I was able to figure out how to fix this shortly after my submission. The purpose of my post was to call attention to this rough edge in the FMCloud experience and leave some breadcrumbs for others who encounter this issue.