You can activate a lookup value of a lookup list in the Lookup Settings tab. The lookup list itself cannot be deactivated.
Activating a lookup value:
1.
Click the Lookups tab.
2.
Click the lookup value that you want to activate.
3.
Select the Lookup Settings tab. A list of lookup values is displayed.
4.
Select the toggle button next to the value you want to activate.
5.
Click Save.
Scenario: in this video, the "dress-shirt" lookup value is activated by a user.
Result: The lookup value is activated. This value is now available in the lookup list and can be selected when creating or modifying a product that use this attribute/lookup combination.