Use “Layer 3 Firewall Rules” on the MX
Go to Security & SD-WAN → Firewall & traffic shaping → Layer 3 firewall rules
Add a rule to block traffic between Wi-Fi VLAN and LAN VLAN:
Policy  Protocol       Source                           Destination          Port/Range
Deny   Any             VLAN 20(Wifi VLAN)     VLAN 10(LAN)        Any
Make sure this rule is above any allow rules, because Meraki applies rules top-down.
This will prevent Wi-Fi clients from reaching LAN devices (including RDP, file shares, etc.)