I am not quite sure what they are actually threatening to do, but "the Haredim" are threatening to somehow shut down the Jerusalem Marathon this Friday if the yeshiva student arrested this past Sunday for being AWOL (as a follower of Rav Shmuel Auerbach and part of the "Peleg Hayerushalmi" he avoided all the IDF notices and did not even do the initial registration with the IDF) is not released immediately.

They seem to be particularly upset this time, as if the previous times boys were arrested they were not really so upset, because previous arrests were made randomly, when the authorities happened to stumble upon the relevant young man, while this time they planned his arrest and got him at home while he was out of yeshiva for Purim break after he successfully evaded the IDF arresting forces for over a year.

They consider this an escalation, though I am not sure why. Because they planned it and looked for this guy? I am not quite sure how somebody evades arrest for this for over a year. If he was learning in his yeshiva. How hard could it have been to find him?

Anyways, they say that with the marathon event requiring much of the city roads to be closed down anyway, and to prevent any disturbances they will have to deploy an immense number of police and security forces, along with dealing with traffic disruptions all over the country, and their protests will exasperate the disruption of traffic.

So, basically, they are saying they are going to hold some more protests at various major intersections in Jerusalem and in other cities around the country, and that will "seal the fate of the Jerusalem Marathon".
source: Ynetnews

Do they think the Israel Police does not have enough forces to deal with their additional disruptions? Even if their upcoming protest would be wildly successful, what fate would it seal for the marathon? Do they think the police will shut down the marathon because of some protest somewhere holding up traffic? The issue might be easily resolvable - divert some marathon runners, especially female ones, through the protests and have them cause some disruptions of their own...

They are a bunch of delusional loons who think they wield more power than they actually do.