donkey dupe

So a player found a way to dupe any item using donkeys and mules. i run a 1.20.4 server that has viaversion so players from 1.21 can join, the three slots that show up without putting any chests on them can easily dupe items. how do i fix this? should i just delete via or is there a way to get rid of donkeys or spawn them with chests already on them? (doesn't work when theres chests on the donkey or mule)
Thanks :)
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