if you don't want to give your designs away for free - don't post them on thingiverse :-)
It's not rocket science.
Just like if you don't want nude pictures of yourself on the internet - Don't be stupid enough to store them online and don't blame anyone else when they pop up on websites. (ah hem, sorry about that - I get very annoyed at those who blame other people for the stupid things they did themselves)

If you want to sell your designs - use 3dsha.re or my minifactory or any of the many others that seem to be popping up all over the place.