Hi,
Each downloaded shows, including the previews, are located in C:\Users\[YOUR_USER_NAME]\AppData\Local\vghd\models (at least on windows).
In this location you will find 1 folder per show, and you can pretty easily identify which folder correspond to a specific show accordind to the name of the folder (aka the the show's number, which is displayed on the cards in your iStripper app).
From now, you just have to identify the preview's folders and delete them. I don't think of any other method, sadly :(
I hope I have been clear :)