Deactivating a user will inhibit them from accessing the team they have been deactivated from. They will not be able to see files or chats nor will they be able to send new messages or see any new messages sent. They will have no access to the team until an Admin reactivates the user.
Reactivating a user is as simple as deactivating them. As an admin simply go to the contact you want to reactivate, click on the cog next to their name (HINT: It should say DEACTIVATED in red next to their name), scroll down to the blue box that say “Reactivate User” and click it thus reactivating the user. The user will then have to restart their app to regain access to chats and files they had sent previously. The user will have to rejoin any channels they had been in prior to deactivation.