Many Mac users have updated to OS X Yosemite but there is a small group who don’t want to update their Macs. If you are wanting to stay with your OS X, you should probably hide the Yosemite update so that you don’t accidentally install it.
If you choose to hide Yosemite update, large OS X Yosemite banner will be disappeared from your App Store. So the large banner is no longer taking up most of the “Updates” screen in the Mac App Store.
This change isn’t permanent situation, you can reverse it any time.
How To Hide the OS X Yosemite Update
This change is reversible. If you decide to change it later tab “Purchase” again and select the Yosemite update. This will start the installation process.