To delete a user account
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If Campaign permissions are set up in a way that restricts ownership or access to a Campaign object to a single user, deleting the account of that user makes the object inaccessible. Instead, you should disable rather than delete such accounts.
1.
Click Settings > Users.
The Username list is displayed in the left pane.
2.
The user account details appear in the right pane.
3.
Click the Delete User button above the account details in the right pane.
A window displays the message, Are you sure you want to delete this user?
4.
The user account is deleted, and the user name is removed from the Username list.