If someone goes around spouting prophecies in the name of an idol, we are not to give him any credence by asking for more information or for proof. We should cut him off, stop him altogether if possible, not encourage him. You wouldn’t ask a thief or a murderer to keep on committing his crime – why spur an idolatrous “prophet” on in committing a capital offense?
The reason for this mitzvah should be pretty obvious. We’ve spoken before about how horrible idolatry is. Shouldn’t it go without saying that we shouldn’t encourage such things? What good could possibly come of it? All that happens is that we increase the possibility that someone might be misled by their words.
This mitzvah applies in all times and places. It is discussed in the Talmud in the eleventh chapter of tractate Sanhedrin (see, for example, the actions of Gechazi described on page 107b; I think those would qualify!).This mitzvah is codified the Mishneh Torah in the fifth chapter of Hilchos Avodas Kochavim. It is #28 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #22 of the 194 negative mitzvos that can be observed today as listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.