How can I hide an Application from the Updates Tab in the Mac App Store?
Go to Purchases in the Mac App Store, locate the Application you want to hide, right-click on it and choose Hide Purchase.

After a clicking the Hide Purchase button in the prompted screen, the item will fade and you no longer will receive update notifications of that Application. 

To unhide it just go to Mac App Store / Store / View My Account, then click on View Hidden Purchases from the Account Information Page, and click the Unhide button.

In both cases you'll need to be logged in.