Hi Chris,
My checkout time is also 11AM. At 11:05AM, I have an auto message through Airbnb send a 'thank you for staying with us, we hope you loved your time and will come back...'. If my cleaners arrive before the guests leave, they have instructions to knock on the door and get the guests moving along. Airbnb won't support a fee to the guest staying longer unless you have it in your listing or rules but if you do, and it was an inconvenience for you or your cleaning team, I would enforce the rules you've created. I always kill with kindness so in my late check-out fee message, I will say something like, I love that you enjoyed your stay and wanted to stay longer! Or, we understand things happen while packing up that may delay a check-out...unfortunately, our cleaning team charged us a fee as they waited to clean the property and we're passing this on. We hope to host you again in the future!"
If they give a retaliatory review, I would call Airbnb to have it removed.
~ Lindsey