Blocking upstream dns for VLANs

I have serval VLANs which are only available on LAN site of my router running Openwrt 23.05 and are blocked by the firewall from WAN access.
The clients in the VLANs can sucessfully resolve local .lan adresses by the internal DNS Server of the Wrtrouter.
On WAN site of the Wrtrouter I have a rapberry pi with pi-hole running as DNS Server.
In the pi-hole query log I can see also DNS requests from clients in VLANs which have no access to the WAN and are blocked by the firewall.
As I understand it, this is the case because the client in the particular VLAN sents the request to the local openwrt DNS server and that forwards it to the upstream DNS server (in my case pi-hole)
Is there a way to block the forwarding of "non-local" DNS request for a particular VLAN, or does the local DNS server always forwards all requests which are not local to the upstream DNS server?
In other words, I like to have VLANs were the clients can resolve only local .lan adresses, but were external adresses are not resolved and not forwarded/ / requested to the upstream DNS server.

Thanks, Mark