Wait until they've had their fun for a while, then recover their password, go into their account, change the password thus locking them out, and have a long good root through all their stuff looking for blackmail material.
If not, then leave them locked out of the account and post some embarrassing stuff on their profile because serve them right.
If I do find something, then I'll screenshot it, get in touch with the user and blackmail them. I don't know what I'll blackmail them for. If the material is good enough, it might even be money. Otherwise I might make them do something time-consuming very specific for me, tailored for my convenience. Stuff like have a look at all my likes and dislikes, then find, watch and send me detailed reviews of new anime they think I'll like. Get them to commission the best DeviantArt artists to draw lots of fan art for my favourite franchises, or to collect stories they think specifically I will like, print them, bind them into hardcover form with a beautiful cover design, and mail them to an address I can get them from. Oh, and I'd send them a TON of artworks that I'll make them get printed as fabric wall posters, all at their own expense of course, and mail to the same address. If not even ONE little detail is to my liking, I'll make them do it all over again until I'm satisfied, or I'll show everyone on the site their darkest, most embarrassing secrets.
I'm just kidding. You don't think I'm THAT vicious, do yooouuuuu?