Hi,
Thanks for using our plugins. I can help out here.
By default once you delete an event that was imported by the iCal Importer it still remains in the database so it is not imported again.
I found this plugin will delete those events:
Optimize Database after Deleting Revisions
Optimize Database after Deleting Revisions
Make sure in the settings you click Delete trashed items before running the optimizier.
There should not be any issues with this, but it is a good idea to backup your database before running it.
Also please note we answer support questions Monday Through Friday.
Thanks