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.
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.