Bill Cornett , May 06, 2007; 08:50 p.m.
Kari-- First of all, if you are located in the US you should know that it is illegal in the States to remove a copyright notice. That can get a copyright violator worse off than just for minor infringement. However, if the edge is cut off the problem is proving it, and in that your information is correct.
You can get a rubber stamp at just about any office supply store. They can have them sent out to be made custom, or you can get a make-it-yourself stamp kit for about $20 or so. Just be aware that most rubber-stamp ink is very liable to smudge or be wiped off of the back of most printing papers, especially if you are still using the kind that works with a darkroom and chemicals (i.e., resin-coated).
You are better off going to a custom print shop and getting a roll of labels with your copyright and contact info on them, backed by "security" adhesive. This type of label is very difficult to pull off of whatever it is stuck onto, and the harder the miscreant tries to pull it off, the more it is evident that a label was tampered with. In some cases, the paper will rip before the label will release.
a much cheaper method is to sign the back of the prints with a Sharpie permanent marker. Just test it to be sure that the ink will not seep through the paper before you do it for real.
The method I'd use, though, would be to watermark each image with a copyright notice. It's time-consuming, but worth it if you are that concerned about getting your images stolen.
Happy shooting. -BC-