Try to remember which sites you registered on in the past. Did you keep an online journal when you were a teenager? Log in with your old profile and delete it. If you’ve forgotten your password, every site has a way to retrieve it (usually by simply providing your e-mail address). You can also search through your e-mails to find the original registration/confirmation letter from the site. In fact, searching through old e-mails is an effective way to remember which sites and forums you previously registered on.