CV's method is a simple solution that will work and doesn't require you to know any computer skills.
If you have fairly good computer skills, I can send you a more detailed method.
Basic idea:
Archive all of your models to an external HD. you can skip this step, but don't recommend it.
for Each model you don't want.
delete all of the .vghd, and .demo files
If you only want to KEEP a small number your cards, then (only if you made the archive)
Delete ALL .vghd and .demo files - Clean them all out...
then just copy from your archive the .vghd and .demo files for the cards you want to keep.
Each Models folder has two files in it that tell VG the card has been downloaded.
by keeping these two files in the models folder, that card will never re-download unless you manually Delete it and re-download it through the GUI.
It will however still show up in Your My collection Screen.
Because once you purchase a card, it is in your collection always.
So you can reduce the space used by the unwanted cards, but you can't really remove it from view.