Would what he is doing count as tampering? I don't remember the details from his meeting with the candidate, but funding/donating to the campaign isn't likely illegal, as well as using his media resources.The obvious way to bring him down, will be someone getting evidence, that he's tampering with the Mayor's election. The most likely person to do that, will be Lena herself, because Seymour has already kind of admitted it to her, when they bumped into the candidate he's supporting. Whether Lena will do that ultimately, depends on how you play her with Seymour. I suppose Ian might be another possibility, if he gets closer to Seymour by winning the Literary competition, or rising higher at the publishing company. Ian's moral compass, is potentially higher than Lena's, and he may feel compelled to reveal any evidence of illegality on the part of Seymour, particularly if he has a high relationship with Emma.
All they'd need to do, was show any evidence to Mayor Vermeer, and no doubt he would do the rest
I can imagine Seymour using Perry to somehow disparage the Mayor, in addition.
Ian is too detached from Seymour at this moment (unless he goes as protege in the future) to catch blackmail, but Lena can plan a double-agent and get documents/tape the room/etc to get serious ammunition. However, illegal evidence is not viable in court, and Vermeer is by-the-book type.
Also, regarding Seymour drying their sources of income, both have options. Billy strikes me as a cryptobro, who made good money out of Bitcoin and now spending it on what strikes him fancy. While the game clearly doubts his plan (I'm getting Entertainment 720 vibes), he might have enough dough to make a decent portfolio for Lena. Also, there are hooks about the music tournament.
Ian has Victor White, who is starting his internet publication. If it kicks off, it can help with self-publishing his book and give Ian a decent job.
The idea of the old money (Addingworth and Seymour) unable to influence the Internet (OnlyFans, digital publishing, crypto millionaires) kinda works there - for how wealthy and influential they are, they are competing with literally the whole planet.