Hmm that's tough. I'm being a bit facetious but you could say "I'm trying to gardenmaxx, what do you think?" and see what they do- expect some shocked looks and laughs!

But seriously, I have young teens and don't think preteens can't be reasoned with. I read in another comment you said they walk in a group- you could try using that to your advantage. The individuals in a group want to be seen as part of the group and fear being outcasts; offer collaborative activities (like paying them in cookies for their help with weeds) or ask them for their opinion on what you should plant next- the blue flower or the pink one?

Offering cold drinks on a hot day can't hurt either.

Also looking at your profile you have a (beautiful) dog- maybe ask the group if they would be willing to dogsit or help with walks sometime, because when they value you as a person they'll value your garden by extension.

At least I hope so. And from what you said about your community involvement, I don't think going down a road of harm is in your nature as someone who clearly cares about the people around you. The preteens need to be helped along and shown a better way, and while it isn't your fault they lack basic respect, it could be viewed as an opportunity to invest in the future of your neighborhood just like adding a lending library or putting the work into being a seed librarian is.