In order to remove the user, you have to be logged in as an user with Admin permissions. Then you'll be able to deactivate and remove a particular user.
See below for details.
Go to Accounts management page
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Go to Edit user details link
Click Deactivate option to deactivate the user
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As soon as User account is deactivated you can activate it again or remove completely.
Remove user
When you deactivate user, option to remove user will be visible.
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If you choose to remove user then you have to select new owner of the presentations and other contents (e. g. Tags or Orbittour Templates) of the removed user.
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The user account will be removed and all the contents from this user will be moved to the new owner.