Sending a Computer for Repair on Tisha B'Av

Due to a malfunction in my computer, the warranty service is supposed to come and take it for repair. It seems that they will come tomorrow on Tisha B'Av. (It's not under my control. I tried calling them; there's no one to talk to.) As far as labor is concerned, there is no prohibition by the letter of the law, (refer to Remo, Orach Chaim, 554:22, “Where it is customary not to labor until noon”). The question is whether in such a case, when the messenger who comes to take the computer is Jewish, one should be stringent and not answer his calls, thereby preventing him from doing work that is prohibited by custom. (But if the messenger is not Jewish, it is permitted even if he does labor for a Jew, as explained in Shulchan Aruch there).

Shalom!
It seems that you are not doing any work here. You have no obligation to prevent “placing stumbling block before the blind” as regarding the messenger, who will likely just go on to service other locations anyway.