Recently, I've become aware of a lot of local discussion concerning the kashrut of Scotch and whiskeys. As a service to the community, and without presuming to pasken (since that's the purview of the Rav and Lishkat ha-Rabbanut), I did some research. The results are available here and here (the latter is far more detailed).
The bottom line is that certain types of food raise potential questions, and there are is a broad spectrum of legitimate halakhic response to those questions. Take, for example, the use of gelatin. In the US and parts of Europe, products containing gelatine are not used. [In the US, by the way, there were Gedolei Yisrael who allowed gelatin derivatives if these were less than 1 in 60 of the total, even if these received them as a matter of course (ie לכתחילה).] In Israel. and other parts of Europe, their use has been allowed since the 1930's. Here, in Israel, this was the view of the Rav of Yerushalayim, R. Zvi Pesach Frank zt'l and is widely held (especially in the candy industry).
The same type of situation is true for Scotch (and other whiskeys). Scotch is cured in vats that may or may not have been used for non-kosher wine. The Posek ha-Dor, Maran R. Moshe Feinstein zatzal, paskened explicitly that liquor manufactured in that fashion is absolutely kosher. It is true that he, himself, tried to be mahmir. However, when required, he himself drank Scotch that was prepared in the above noted manner. In other words, one may be mahmir without casting aspersions upon the essentially permissible nature of an action or a foodstuff.
And that, Ladies and Gentlemen, is the point. Most kashrut issues lie along a spectrum of permissibility. There are acknowledged Gedolim who differed sharply on such issues, and those who accept their authority are fully within their legitimate religious right to rely upon them. No one, absolutely no one, has the right to tell them that they are (God forbid!) eating trayf.
On the other hand, it is often the case that individuals are unaware of the realia of kashrut. Under those circumstances, it would be the proper thing to quietly take the person aside and explain that some people don't accept a certain hekhsher for this and this reason. It would then be up to the individual to decide. However, propriety dictates that (if applicable) both sides of the issue be responsibly presented (as well as the halakhic authorities who assert either side of the question). Anything short of that is, frankly, both misleading and abusive.
The Torquemadas of Kashrut often recall lists of forbidden ingrediants, but never read the introduction of the Netziv from Volozhin to Humash Bereshit. The Netziv (R. Naphtali Zvi Yehuda Berlin) zatzal, asks Rashi's question: Why did the Torah commence with Bereshit and not with the first mitzvah? His answer was 'Derekh Eretz Kadma la-Torah.' First one needs to learn to be a mensch. Only then can one start to accept, understand and observe mitzvot. As proof of his contention, he notes that he Second Temple was destroyed because pseudo-tzaddikim beat each other up with their frummkeit, and (as Netziv says) "the Holy One, blessed be He does not suffer such 'tzaddikim.'"
After Tisha B'Av, and in advance of Rosh Hodesh Elul, it behooves us to improve our devotion to mitzvot. It's easy to do that by crusading for humras, to check other people's tzitzis, and to carry one's own frummkeit as if it were going to break. It's easier to feel good by humiliating someone else under the guise of saving them from sin.
It's harder, more valuable, more legitimate and more worthwhile to acknowledge the fact that there are different traditions on various things and that, in the absence of the Sanhedrin, this type of limited pluralism will be part of our lives for the foreseeable future. My master and teacher, Rav Soloveitchik zatzal, refused to tell his disciples and laypersons what his person strictures were. These were a matter between himself and his Maker. A person who violates the privacy of his personal piety, in order to impose his standards on someone else who relies on his own rav is, frankly, is nothing more than a bully.
We have enough bullies in the religious community.