What kind of thorny plant / shrub is best under a shaded window for security purposes? Zone 8!?

How about the barberry. They come in different varieties. We have both, a burgundy leaf with lots of thorns and we have a golden color leaf with lots of thorns. They both flower with tiny white blossoms. Both are growing in part sun and shade. It's considered a medium height bush. Worth trying, we really enjoy ours. After flowering, you can cut them back to the size you prefer each year.