Growl will notify you based on what other apps tell it to notify you about. So if you look in the Growl history, you can see what app is notifying you. Search for that app in the applications tab, and go look at the Notifications tab to see if you can configure that specific notification.
If it's not there, ask the developer of the app (assumedly Hazel) to add a notification for that specifically to the registration dictionary.
When I delete an application, Hazel shows me any associated files and allows me to delete them. At the same time the Growl notifications window open and also offers to delete them - how can I stop Growl doing this? I can't see any parameters in preferences to do this.
Any help/advice appreciated.