You can refer to this post: LEDE/OpenWRT — Restricting Network Access Based on MAC | by CT WiFi | LEDE/OpenWrt & IoT | Medium
and this OpenWRT: Create a WiFi MAC Whitelist or Blacklist
This can only be done by editing the file directly.
option macfilter 'deny'
option maclist 'xx:xx:xx:xx:xx:xx yy:yy:yy:yy:yy:yy'