To deactivate a user, you just need to access the user profile in the Settings page.
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You then just need to click on the "Deactivate" button and press "Ok". 

Deactivated users won't have access to Claimable anymore, and they will be immediately logged out of their account and unable to log back in, until you reactivate them.

Only users with the role "Admin" can deactivate users.