scenario: you know a chud at your workplace. he shows you a 'funny' meme on his phone and you peep his twitter handle. it's LoliHitler88. you hate this guy so much. can you put his real name and face on a billboard next to his greatest twitter hits if you don't call him any names on it. is that considered harassment or free promotion. this is a hypothetical
Spitballing here...
Unless this person is a public figure there's, at the very least, an argument that the billboard company might refuse. Also... you're gonna have to do a ton of legwork to try to keep this from tracking back directly to you.
If the billboard company allows it, that person you're putting on blast can try to take you to civil court for any damages real or imagined. At a minimum, you'd be expected to pull down the billboard plus any court fees you and probably the plaintiff incurred.
Find a public place to print off some flyers and pay with cash, wear a hat and a "loud" scarf/shirt. Drop them in places where somebody may find them around the office, hopefully with enough people handling them to get everybody's fingerprints on 'em.