This should work in the article that Jan mentioned:
Keypad Mouse (Activate/Deactivate)
To set the keypad to control mouse movement, press the Command-Shift-Clear keys. The 1, 2, 3, 4, 6, 7, 8, 9, keys control movement and direction of the pointer. The 5 key is the mouse button. The 0 key locks the mouse button. The decimal point key unlocks the mouse button.
However, assuming that your Classic II has a great deal of your personal documents/info, deleting every single file in this way would be a royal pain. The most secure option would be to boot off a startup disk with a hard drive formatter such as Apple HD SC Setup and initialize the disk. If you would be interested in selling the Classic II to me (I am a collector who doesn't own this system yet), I have an extra ADB mouse and startup disks with HD SC Setup that I could send to you.