@scott for things like these I always emphasize moving the user experience logic to the user interface, including things like controlling which messages the user sees.
Don't control membership to the forum. Let anybody post whatever they want, but just don't show unwanted posts to the person following the forum, based on their own preferences for what they want to see.
Only want regulars or certain registered members contributions to show? Sure! But handle it in the UI so we don't have to go down the road of centralizing content controls.