After you protect a worksheet, you can lock an individual cell to prevent it from being changed. This level of protection goes beyond simply protecting a worksheet; it prevents anyone using the worksheet from changing the contents or formatting of the cell. Excel 2007 enables this option by default.
Lock Cells
Select the cells you want to lock.
Right-click the selection and click Format Cells on the shortcut menu.
Click the Protection tab.
Select the Locked check box.
Click OK.
Caution
You must protect your worksheet after you lock the cells for your locked cells to become effective.