A Serial Number or Activation Key can be locked to prevent misuse.
The key has not been used
If the Serial Number / Activation Key has not been used, you can add the key manually and lock it - see Administration / Setup, Add a Key on the Activation Server. Alternatively you can import a key list and lock the complete key list.
The key has been used
To prevent, that a key can be used again (even for an reinstallation on the same machine) you have to search for the key.
Select Manage Activations Tab, Activation Requests. In the field Entry Key type in the Serial Number and press Search. Then click on View to get the detail page of the record. Check: Is Locked and press Save.