If you ever need to clear all values of a defined property within a hierarchy (prior to blending in a completely fresh set of property values, say), it can be done in a few brief steps.
Firstly select the top node of the hierarchy and choose the tab the property is on. If the top node does not normally show this property, you will have to select Show All Properties on the property viewer. Only administrators can do this but then again, only administrators should be doing this.
Next, right-click on the property label and choose Clear All Below, followed by Save. This removes all explicitly defined values from each node below the selected node. (This is a useful little option in itself.)
Finally, right-click on the property label and choose Remove Value, then Save. This removes the property value from the top node.
There are action script equivalents of these two commands, so if you have a whole set of properties to clear an action script is probably the way to go: