Once a contributor has read the notice, they may prefer not to view it any more. If they have cookies enabled, they can dismiss the notice using the "dismiss" link next to the notice text. This will work whether they are logged in or not. As long as they don't clear the cookie, the notice will remain hidden.
To reset the notice and make it visible again to all (for example when an administrator has put up a new notice) the number in MediaWiki:Sitenotice id needs to be incremented by one. The new notice can be dismissed after reading in the same way as the last.
To clear the message, Mediawiki:Sitenotice should be reset to the default: -