Hi and thank you for using just answer.
When this happens, you can insert a screwdriver into the hole to lock the drive. If you turn your machine on the side, just off from the shaft, about the 4 o'clock position (the front middle being 12 o'clock) there is a hole in the base of your machine. It might be covered up with grass clipping but you should find it. Then stick a screwdriver through the small square hole of the blade and into the base of the machine. This will hold everything, giving you enough leverage to remove the blade. If you have any other questions, please ask. I wish you the best