If someone calls Hatzalah on Shabbos and then realizes that everything is fine, should he call back hatzalah to cancel the call? Does it make a difference if it is in the New York area where hatzalah members drive back after call? Perhaps it is better to call back to prevent what they are allowed to do because of סופו משום תחילתן and by calling back you are preventing all the unnecessary driving after the call.
You should not call them to tell them not to come. The reason is because they are permitted to come, because they think that it is a life threatening situation, therefore they are not doing anything wrong. They are also allowed to go back under the same conditions as any other call. Therefore the only person violating Shabbos here would be you, because you are calling then and it isn’t not in order to save a human life.
Nishmas Avrohom 1-siman 238-1, Shulchan Shlomo 307-4(2), Shevet Halevi 8-193(2). See NIshmas Avrohom ibid that you may tell a gentile to call, however see Toras Hayoledes chap. 23 ftnt.1, that it wouldn’t even be permitted to tell a gentile to call them, because that would be a shvus. If you want, you can tell a gentile to tell another gentile to call.