re: Best legal/semi-legal way to humiliate, piss off, or get under someones skin?

First off you broke rule number one. You don’t ever talk about this to anyone. Second rule is you never talk to anyone about this. The third rule is the same! If you ever want to do dirt you keep your damn mouth shut.

For some of these ideas, you'll need to start another email account that cannot be linked to you. If you're really paranoid or are doing something that could be found to be a breach of privacy (like posting naked pictures without consent to post them) use a VPN for anonymous browsing (or at least a public access computer) so the IP address can't be traced back to you. This is very important.

Make a Facebook profile with the profile picture of the a-hole then add all of his friends and even remote acquaintances and post radical racial stuff particularly if he’s white. Post an adult dating/hookup ad under “men seeking men” or something equally embarrassing for any other gender/orientation so the a-hole will receive a steady flow of colorful calls/texts/messages from friendly locals looking to have a good time. (You have to be careful with this one though, because you can't impersonate anyone by using his/her name or contact information on the actual posting.) Call an adult escort service (search for one in your area if necessary) and make an appointment for an escort or stripper to go to his house at ungodly hours of the morning or night on days you know the prick has off from work and will be home. Request literature from the Church of Scientology, Mormons, any other weird group and give them his name, cell number, work address, home address, and all known email accounts. Order sex toy or condom catalogs be sent to his work address, company email, etc. If you have access to his phone or account (like iCloud, Google, cell service), change the password, then jack up the phone bill with added services. Report the guys vehicle stolen so he get pulled over the next time a cop sees the a-hole driving around (have the license plate and vehicle description ready). Subscribe to every single spammer and porn newsletters with the guys email address, phone number and home address. Send dirt (pictures, arrest record, etc.) to the guys employer, new girlfriend, parents, or other family members if you can find their mailing address or email address. Pretend to be a customer where the guy works, then complain to the manager or file a formal complaint. If you have access to his social security number apply for a cash loan, credit cards, anything you can think of using personal information so that he has a credit mess to clean up.