Fungal infection. These can be a real pain to shift even with modern medicine. Make the fungus show up on the skin of the offender as contrasting coloured or textured lines (so it could work regardless of original skin tone). The fungus needn't be dangerous as such or even particularly unpleasant, though it might itch a bit as it grows through the skin. Just something that people can recognise that can't be trivially removed. Up to you whether it might be permanent or not, or whether it could scar. It may or may not be contageous between humans, but they might believe that to be true and someone afflicted with the mark could be treated as a leper.
Now part B, the fun bit.
perhaps encouraging vegetarianism/veganism in the victim
Ever heard of an alpha-gal allergy? In the real world this is spread by tick bites, and causes an unpleasant allergic reaction to the consumption of red meat. You can certainly run with this idea, inducing a more general allergy to a wider variety of meat and fish or other animal proteins (eg. dairy).
You could combine these two punishments together, with the fungus causing the allergy (and effectively making the allergy incurable) or separate them, with the meat allergy fading over time as a real-world alpha-gal allergy can do. That allows multiple options... temporary punishment, permanent punishment, and permanent punishment with effective branding.
Merely possessing a meat allergy might be stigma enough, depending on the society the offender comes from.
Depending on the sources of protein available, this could be anything from a mild irritation (if you've got good access to beans and oily seeds) to an extremely serious problem if you've got too few protein sources, or the ones you do have just don't provide quite all the nutrients you need.
Now I think on it a little more, it is odd that intelligent plants would seek to punish animals by making them less inclined to hunt, kill and eat other animals. Seems like encouraging your enemies to hunt and kill your own pest species instead of eating fruits and grains might be far more useful and perhaps a little better suited to plant pacifism.
The same suggestions I made above would work just fine, but turned around... induced nut allergies, gluten intolerance, etc. Encourage the victims to eat more deer, beavers, bunnies and so on.
(see also: the green pact bosmer of the elder scrolls series, a neat take on wood elves, who are carnivorous and cannibalistic)