I respectfully disagree, for a couple of reasons.
I'm not advocating a double page spread listing all of the things he's done wrong, that might lead to a defamation case. Something simple like "Dejphon Chansiri, the supporters of Sheffield Wednesday no longer support you, time to sell the club" is fairly ambiguous and makes no accusations that could be misconstrued or contested. Any fund raising for an advert could also pay for legal advice to check the legalities, and likely a reputable paper would run any ad via it's own legal department as well, to cover their own backs.
As successful as the Birmingham protest was, Chansiri is 7000+ miles away the majority of the time, and can safely ignore it in the hope that Röhl keeps us up and the heat dies down. Whilst Thaiger is technically the Thai media, it's primarily English speaking, and the reporter is from Newcastle. I don't know how likely it is to get picked up in the domestic press there. Taking out an Ad would guarantee coverage, and get people talking closer to home and might embarrass him into selling up.