People can do a bug where they get barriers from the gui.

Could you expand on this? What are the steps to reproduce the issue?

I've tried this and many players did as well and succeeded, I would try and explain it but I'm just going to send the video that a players sent me. https://youtu.be/IUYLySgDsHM

Lol I had this issue on my server nearly 2 years ago now. I just added an illegal items check. However i think they can also get emeralds, but you can check for and delete the emerald gui item because it has a special name.

What Illegal items check do you use? I've tried to find one but failed to find one that works on the latest version.

Duplicate of #250

i had the same problem and i found a temporary fix for it,
you just remove the permission to use /maps command, if the player cant access the GUI using this command; they cant get illegal items.
i used Luckperm for that but you can use what evet you want.