Once a fleet is actively rented, you cannot cancel the rental immediately. The borrower has full control until the rental period ends.
However, if you no longer want your fleet to be re-listed for future rentals, you can:
Click “Cancel Future Listing” on the fleet management panel.
This means:
The fleet will not be listed again once the current rental ends.
The current renter can continue using it until the rental expires or they cancel it.