If you know the URL can't you use a replacement variable to censor the text?

Yes you can stop this by censoring the known link.

Preventing that this can be done (ie. you don't know the link they will use yet) is not possible however as you can not determine (or each link posted would need to be test in some way if the image is protected or not) if it is a mallicious link or not.

Bottomline is that they will always find ways to try to trick your members. Only educations is a real solution. Compair it with spam emails, phishing emails (which are similar to what happens here) or emails with mallicious links. Almost impossible to stop, only real solution is to educate people to think before clicking or entering personal information.