Follow these steps to deactivate CUSTOMTOOLS license:
1. Select "Help" menu and choose "Manage CUSTOMTOOLS Licenses".
2. Fill in your email address if it is missing, select "Transfer license" and press "Next".
3. Make sure that you will get a message "Product transferred successfully" and press "Next".
4. Now you are able to activate CUSTOMTOOLS license in another workstation.
Note: Usually you need to deactivate SOLIDWORKS license as well. Check THIS INSTRUCTION ABOUT DEACTIVATING SOLIDWORKS LICENSE.