Robin – thanks for the reply and the additional tag.
I'm not on the MS admin team and have never logged into Power Automate. I don't even know if I have permission to the product, much less what to do if I could log in.
I found the "Schedule a Reply" workflow in the library of pre-built workflows available in MS Teams. One of our IT folks created the "Schedule a Message" workflow, which is very similar, and sent me a copy to install on my Teams account. I just clicked the link and then a button to install.
From the workflows tool within Teams I am able to edit the workflow (definition) and deactivate it to prevent further use. But I'm not able to stop/cancel the specific instance of the workflow that's still running.
All the best...