How to completely remove iMovie and avoid updates
If you hold ALT/OPTION, the Appstore (on the purchases tab) gives you an option to cancel the download (assuming it is incomplete).

Depending on the version of OS X you have additional steps may be required to remove the application support files (in /Library/Application Support/).

Appcleaner can be used as well, it does a good job of finding the dependencies; but to enable the removal of system and default applications you need to change the Appcleaner preferences.



I've never needed these options, so proceed with caution.

There is a debug menu in the Appstore that has several more advanced options for controlling the application. Resetting the application may help in this case. 

Another option, but that it is ideal is to let the update run (possibly reinstall it first) and then uninstall it again.