How to prevent Guest Network clients to communicate with each other?

Dammit, i'm using the optimized build from here: https://forum.openwrt.org/t/optimized-build-for-the-d-link-dir-860l (r6795) cause the dev told me the last build is abit "wonky"...

edit: stable = 18.06 ? Cause wifi isn't working with these version... nvm, that is a snapshot as well.
Are u talking about CC 15.05.01 ??