Sometimes you want just one or a few plugins to never get updated, typically when you have done some customisations to them, which would be overwritten once they are updated. Obviously you don’t want that to happen, so in order to prevent it you can access the plugin’s php file and edit the commented section that is found at the very beginning of the plugin file, change the plugin URI to something else and it won’t update anymore.

You can also use the following function:

http://markjaquith.wordpress.com/2009/12/14/excluding-your-plugin-or-theme-from-update-checks/