I too am having this issue. There where 3 cards I downloaded the previews for and no longer wish to purchase them and I can't find a way to delete them. my main harddrive is getting close to capacity so I'd like to remove these at some point. On older versions it was quiet easy to do so but not since I upgraded.

@v23326445

You don't want previews sent to your system.
See screen shot above.
Follow these steps:
Step 1: find settings icon and left click once (the little gear in the top right corner) see purple arrow in screen shot.
Step 2: Make sure "advanced settings" is checked at upper left of settings window. (Not shown here).
Step 3: Scroll down to "Clips" (Red Arrow)
Step 4: Find "Free Preview Push" (Green Arrow)
Step 5: Click window just to right of "free preview push" to drop down choice window.
Step 6: Click "Never" (Blue arrow)
Your done. No more previews delivered to your GUI.
Follow @Manue 's steps to manually delete any previews you may have now that you do not want.
Go to 'My Collection' tab > ctrl + A > Right click on the bin > delete previews only

We'll soon release version .160 and enable users to control the amount of disk space used for demos (right now it's patched by server at startup to fix a bug with versions up to the current one)
And delete previews, together with more precise whishlists, is scheduled for the next update.

@pumpdude48 The problem after following your walk through is that I am still left with previews of the models that I don't own. Your suggestion will only seem to delete previews of models I own.

I have disabled Free Preview Push, I actually did this before reading your post. So hoping automatic downloads of previews will stop.

However I am still left with the previews of models that have downloaded automatically prior to me disabling free preview push.

Any ideas how to delete the previews that I chose to individually delete.