This issue was raised on the netbeans forum here. Consequently a bug has been raised and appears to have been fixed, details can be found here. The fix says that:
Now there's an editor hint allowing to disable the warning about an unknown
property. The settings is global. Once you disable at least one property check,
there's a new hint at the very first line of each file containing the disabled
property which you can use to reenable the checking back.
Encase you are not that familiar with the hints in netbeans, this suggests that the properties netbeans doesn't like will be underlined in yellow and you should be able to click on them, press alt+enter and then an option to disable the warning will appear (I apoligise if you are familiar, I just added for a verbose answer)
EDIT- Misunderstood Question:
To change what is being shown in the Action Items window, you can apply a filter and/or change the scope of the window.
To change the scope right-click inside the Action Items window and click on scope then choose the option. Use this if the unwanted warnings that are being generated are happening in a different file or project to the one you are trying to debug.
You can also apply a filter to the window to get rid of specific messages that share a common atribute e.g. if hypothetically all of your unwanted warnings contain the String "CSSHackWarning" then you can filter out all warning messages that contain that String from being shown.
This can be done by right-clicking on the Action Items window and choosing filter->edit... then creating a new filter an clicking the keywords tab. For more detail, please see here. (Note: pre NetBeans 7.2 the Action Items window was known as the tasks window)
Sorry for the confusion, hope this is more along the lines of what you wanted.