2. Click Format --> Cells and choose the 'Protection' Tab
3. Ensure that the check box for 'Locked' is ticked.
4. Click Tools --> Protection --> Protect Sheet.
5. Select the properties you want to lock for, and supply an optional
password.
Now when you try to change the cell you will be presented with a message
informing you that the cell is locked.
To remove the protection click Tools --> Protection --> Unprotect sheet and
supply correct password when prompted.