Reducing meat consumption is a critical component of climate change mitigation, and veganism is often promoted as a high-impact individual climate action. Yet, despite its environmental benefits, resistance persists. Anti-vegan rhetoric frequently draws on conspiratorial claims that depict vegans as deceptive actors who exaggerate the harms of meat and manipulate public opinion. Despite the prominence of such narratives, their psychological consequences have rarely been systematically examined. Across three experimental studies (N = 1,783), we investigated the effects of exposure to vegan conspiracy content. Exposure consistently heightened endorsement of vegan conspiracy beliefs. Compared to a control group, participants exposed to conspiratorial narratives expressed more negative attitudes toward vegans, perceived greater symbolic and realistic vegan threat, attributed more negative traits, and judged vegans and vegan organisations as less moral (Studies 2-3). Exposure also increased support for punitive actions against vegan organisations. At the behavioural level, participants were more likely to sign a punitive petition against vegan organisations, an effect that emerged indirectly via heightened conspiracy beliefs (Study 3). While exposure did not directly reduce intentions to lower meat consumption or support meat-reductive policies, it increased defensive perceptions of meat (endorsement of the "4Ns": natural, necessary, normal, and nice). No evidence emerged for an effect on food system justification. These findings extend conspiracy theory research in the domain of diet and climate action, showing that anti-vegan conspiratorial narratives heighten perceived threat, foster negative and punitive attitudes, and reinforce meat-defensive ideologies—potentially undermining public support for high-impact climate strategies.