I understand what you are saying, and that once google has followed a link it is difficult to tell it not to follow it any more.
Google will at some time revisit the pages where the links are placed, and will attempt to follow the links from that page. But it is after all only a machine, and although it may have followed the link before, having now made it a 403 if it tries to follow it again it will find it broken.
I have a number of pages on my site which are now 404 because I have deleted them. After trying the links to them a number of times Google will eventually de-index them. The same theory applies for 403, except that instead of de-indexing the page the links are pointing to, it can only de-index the link itself because it does not now know where the page is.
This theory may not work, but the point is that we are very limited in what we can try to do to disconnect our sites from links that could potentially be detrimental. I have not found any evidence that this kind of action can cause any harm, so for me it is a case of doing something rather than nothing and if it helps then it is a bonus.
Last edited by ozsubasi; 24Sep2012 at 11:36..