The "little guy" as you call him/her, is susceptible to emotional blackmail because they have a conscience! If you were completely immune to this kind of manipulation, you would be a sociopath. Part of the problem is that this can only be accomplished by somebody you care about, such as a significant other or parent. As a caring human being, you should be able to trust them to be honest with you. The problem is the blackmailer's, not yours. Hopefully, karma will bite them for it, eventually.