If you are trying to disable a user, and are receiving a reassignment error message, this means the user you are trying to disable is still enrolled in a lead distribution rule set up in Nutshell.
To move forward with disabling this user, go to Setup>Leads>User assignment and remove this individual from any automatic user assignment options. Once the user is no longer enrolled in this rule, you will be able to go back to the Users & teams page and disable this user from Nutshell.