I think the only way is to do a "Master Clear", which clears all user-selectable options, the Phone Book, and the messaging memory. This is from the manual to my ST7867W, so your mileage may vary.
Press FCN, then diamond (the center rubber button).
Scroll to Lock Secure and select with diamond.
Scroll to Secure Options and select with diamond.
Enter 6-digit secure code.
Scroll to Master Clear and select with diamond.
You will see Clear? Press diamond to confirm.
Press End to exit.