How do you know it is not clearing the memory? Is the storage bar still partially full under About? If the storage is not clearing when you are syncing there is likely bad data on the watch and you need to do a factory reset to clear it out.
To do a factory reset, make sure the Connect software is running and dock the device to your computer. The Connect window will open; press the gear at the top right and select factory reset from the next screen. Follow the steps provided and when prompted to register or sign in to MySports, select sign in and use your existing account credentials. It is fairly straightforward and should take less than 5 minutes.