Hi Lewis,
The inspection rules cannot be deleted. They can only be made inactive.
The original reasoning was that if you were doing some forensic investigation and a rule was no longer on the system then the database would be corrupt.
You are not the first person to ask for this and I believe Cisco is considering this feature request.
In the meantime, when you need to make a change to a rule just clone the original and then make the original inactive. In this way if you make a mistake you can make this rule inactive. You can then clone the original rule again and fine tune what you are trying to accomplish.
Hope this helps.
Paul
