Apple - How can I opt out of updates for one app when using the App Store?

While I'm not abosultely sure, how achieved my result, but after a reboot the update for one app-store app is gone now. What I did:

• the following is tested on old Blackmagic Disk Speed Test.app (free from appstore), before my tests the app-store showing an available update for it.
• Right click on app, select Show package contents
• go to "Contents"

• Move the following files and folders to some backup place

  • _MASReceipt (folder)

• reboot

If that doesn't work, remove these additional items from the package contents: - _CodeSignature (folder) - CodeResources (alias)

Maybe it is not enough, (in my tests i did the following too), try:

• Go to Library (alt-click on Finder's Go menu, select Library)
• Caches
• move com.apple.appstore folder somewhere else(backup location)
• reboot

After the above steps, the AppStore don't showing the available update for the "Blackmagic Disk Speed Test.app".

Warning: all the above is tested only in one app, needs more testing, it is absolutely without any warranty and don't blame me when your computer will burns, gets grabbed by aliens or anything other...

I typically have Macs that allow App Store and Signed apps to run by default, so I used jm666's answer and pared it down to one step:

1. Delete the _MASReceipt folder from the Contents folder inside the app.

I have backups, so I'm fine just stripping things from the app's contents and testing. This has worked for me on 10.8.2 through 10.8.4 but could break things in the future so use this answer with that caution in mind.