There is an umbracoVersionCheckPeriod
key in appSettings
section of the web.config
that has the default value of 7. Set this to 0 to prevent "a new version is available" balloon.
<add key="umbracoVersionCheckPeriod" value="0" />
http://our.umbraco.org/wiki/reference/webconfig
EDIT:
For umbraco version 7 this key might not be present in the web.config by default but you can add it manually and the notification will disappear.
http://our.umbraco.org/documentation/Using-Umbraco/Config-files/webconfig7