Indeed, you normally do not need to clear an individual memory: you can just overwrite the one that you don’t need anymore.
If you want to start from a complete empty bank (all buttons blank), you can save your current bank to usb and then use power on + B key to erase all memory pads (the bank on usb still remains available for loading whenever you need it). Personally, on my SX700, I have saved such an empty memory bank (I called it “NEWBANK”) so if I want to start from scratch, I just load this bank and when the individual memories are filled, I then save it using another name, so that I always keep the empty NEWBANK.