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...