Reporting would be the best option, but unfortunately Google doesn't (currently) have this facility. In this type of instance it only advises that I should contact the webmaster of the site concerned, and there is no suggestion as to what to do if that fails (if anyone else knows otherwise that information would be greatly appreciated).
I would be telling Google that my site is not accessible, but only from specific urls? (I should explain that I am not envisaging blocking 100's of urls, maybe 10 or so.) A couple of months ago I noticed a 403 error in GWT, and as it is a server error I contacted my hosts to ask what the problem was. They replied that they did this as a matter of routine where they suspected malware or hacking attempts, and that I need have no concerns. If it is ok in these instances, would it not be if I wanted to prevent access by certain domains for other reasons?
This is my other major concern, i.e. is there any value in doing it anyway, especially if there are risks involved? But it isn't because I just don't like the links, but because I feel they could be potentially harmful.
Users never click on these links anyway, the sites where they placed have no relevance to mine and there is no reason why anyone should have any interest in them. The sole purpose would be to render the link useless, so that for whatever reason it is there, it cannot fulfill the objective.
I was (perhaps optimistically) thinking that Google would maybe investigate those sites which are being blocked.
Thanks very much. I appreciate your reply and that you are only commenting, and this reply is in no way a criticism of your thoughts (which largely echo my own) but merely an exploration of them.