That is a good resource but does not change the user back
I use a particular style for Ad-Free Membership Subscriptions and I want the user to be placed back in the default style when his subscription runs out .. This Add-On you linked will only allow those in that group to use it but when the Subscription expires it does not automagically place that user back in the default style (Non-Adfree) even thought the expired subscription takes the user from that usergroup
What's creating the space? If its CSS you can give the spaces a class and wrap a conditional around the class declaration in the div. With the class not declared the CSS is ignored. Its really dependant on how it was done but it can be done. Since what your looking for is basically the opposite approach solution.
The user stats , people online etc. is what creates those gaps...
Normally I would take a skin and disable the sidebar for the Ad-Free users because all they mainly want is a fast wide forum.. Now being able to use conditionals on the sidebar this kills 2 birds with one stone..
Not doubting you, this is just the perfect solution for me.