This doesnt eliminate Print Screen.
If it bothering them that much, there are basically three solutions.
- Have the images on the site and ignore the fact that they are stealing the images.
- Raise the issue with them personally, and take it further if necessary (if you can prove loss of earnings etc)
- Don't use any images.

If they are hotlinking, you can prevent this, but if it is displayed, they will cache the image or just do a simple print screen.

I would personally confront them about it.

Craig