Hi and welcome to the forum!
You can disable the actions in the state, so just disable them all in one particular state and then the state will be basically inactive.
Or otherwise, create an empty state and transition to and from that, or just don't transition to the state you want to disable.
As for actual FSMs they can be enabled and disabled just like any other componenet, in the game object's inspector.
Hope this helps, but what are you trying to achieve anyway?