To hide a custom action, you must know the identifier of the action you wish to hide. For a table of default custom action identifiers, see http://johnholliday.net/resources/customactions.html. For code you can use to obtain the custom identifiers that have been deployed to the local farm, see http://johnholliday.net/post/2007/10/28/How-to-find-lt3bCustomActiongt3b-identifiers.aspx