Joseph & laskern, if a competent rov holds that a piece of meat is kosher, and you hold it’s treif, you of course can’t eat it, you’re entitled to try to persuade him that his shita is mistaken, and you’re entitled to try to persuade those who follow his psak that he is mistaken. HOWEVER you may NOT impugn the observance or ne’emonus of those people or of the rov himself, you may not refuse to accept their eidus, and you may not refuse to eat in their houses, even from their fleishige keilim!
Therefore the same applies to eruvin. If you disagree with the psak on which an eruv is based you may not use it, and you may try to persuade people that your opinion is correct, but you MUST accept that those who use it are shomrei shabbos just like yourself. It is strictly forbidden to treat them as mechalelei shabbos, or to rebuke them for not accepting your opinion, since they are just as entitled to their opinion as you are to yours.