Select the Addresses tab.
Select the IP address's network.
Select one or more IP addresses you want to mark unusable.
If you want to mark more than one address unusable, press the Control key while you click with the mouse to select multiple addresses. You can also press the Shift key while you click to select a block of addresses.
Choose Properties from the Edit menu.
The Modify Addresses dialog box or the Modify Multiple Address dialog box opens.
If you are modifying one address, select the Lease tab.
Select Address is Unusable.
If you are editing multiple addresses, select Mark All Addresses Unusable.