You out debate them on public platforms.
On the media. On the internet. Everywhere.
You systematically begin applying INCREASING economic pressure and sanctions on the board. Eg boycott of service.
You tell Kenwright politely that small time thinking will see us go back wards over the next three years whilst Spurs and West Ham have spades in the ground.
If Kenwright is indeed ill. Then thats even more reason he should step aside. As he's no longer fit to carry out his duties.
All pretty pointless.
The only real way to put pressure on the board is to find a person, or consortium, who are willing to offer a reasonable price for all, or part, of the club, and work with them to ensure that any reasonable offer isn't rejected out of hand.
To get to that position you need to have contacts which, in the main, are not available to the vast majority of fans / posters / pressure group members.
Given that seems unlikely, then pressure groups need to find a way to convince the board that they can offer something to help constructively, otherwise it'll just be hot air. I don't know how that could happen though.