Start spamming far-left YouTube videos to the public channel at the same time, according to the general theory of nutball political physics the two should cancel each other out
In MeshCore a concept of regions has been recently introduced - you can scope channels and messages to a specific geographic region (which is set on a repeater) and it will not propagate outside of the given region.
That way local channels don't need to flood the whole mesh, same as with trying to send a message to someone or reach the management interface of a repeater you know is in a given area.
The problem is, all repeater administrators in a region need to cooperate - and a lot of repeaters are abandoned. And it doesn't help at all if the troll is aware of regions.
In practice at this quite early stage of MeshCore development I would say that repeater need some regular maintenance (like the recent firmware update needed 1b -> 2b) so if they are abandoned, they will eventually fall out of the network.
As for trolls and regions - based on how you setup the regions a troll might need to physically travel to cause wider issues, as their traffic might no longer propagate through the whole mesh.