Deleting iPhoto events in iMovie

I do want to delete my iPhoto events / movies from the list in iMovie. For some reason I can only delete events / clips that were imported other than from iPhoto.

Does anyone know how to get rid of them? I checked the filestructure to possibly delete manually, but I cannot pinpoint files in either the iPhoto package (which increased tremondously after the initial import) or other places were these thumbnail files etc. are storted.

Anyone got a hint how to go about?

Note: I am aware that getting into the files manually poses a risk and I do keep backups to restore... in case. So risky hints are also welcome 😉

Dirk