A user in 7Targets needs atleast one default assistant. And if you are trying to unassign the user from a default assistant then it will fail. If the purpose is to deactivate/remove a user from the org because he may have left your organisation then “Remove from Org” option is available.
When a user leaves an organisation, you can remove that user from 7Targets too by using “Remove from Org” action under Organisation->Org Users.
It is demonstrated in the following image.
This action lets you assign the leads of this user to some other user in the organisation, like handing over those leads to some other user in the org.