If a Jew is making crank calls to somebody’s house, can a person tell the police or phone company? Is it mesira? You have told the person to stop.
After the person is warned, the phone company may be informed, or even the police.
Tzitz Eliezer (vol. 19, no. 52) writes that the prohibition of handing over somebody to the authorities only applies when one hands him over on account of his very wickedness. When, however, he is handed over because of the threat he presents to potential victims, the prohibition would not apply. This principle is expressed by Rema (Choshen Mishpat 388:7), who states that when a person’s property is threatened, it is permitted to inform the authorities of the threat, and allow them to deal with it—even if this will lead to punishments that are not sanctioned by the Torah.
This would also apply to the harassment of somebody who continually makes crank calls, and does not stop even after being warned.
A similar principle is found in Kovetz Teshuvos (Rav Elyashiv shlita, vol. 1, no. 188), who explains that the concern over turning a person in to the authorities is that “should he confess his crime, they might sentence him to death.” This concern does not apply today, and it is therefore permitted to use the authorities for purposes of investigating the accused criminal.
A similar idea is expressed in Aruch Hashulchan (388:7), who writes that the stringency of handing a person over to non-Jewish authorities applies primarily to decadent countries, and not to enlightened countries in which a person receives a fair hearing.
Therefore, if there is no other way to deal with the issue, it is permitted to hand the offender over to the relevant authorities.