I need a way to block any DHCP requests being "forwarded" by the router onto a PPTP BCP bridge interface
I want local DHCP requests to be answered by the local router only, and not one on the other end of the PPTP bridge tunnel

If I drop ports 67-68 in the forward chain, and select use IP firewall in the bridge settings, will this stop the DHCP requests from going out to another DHCP server on the other end of the PPTP BCP bridge.?

the PPTP BCP interface is in the same bridge as the local LAN/WLAN

DHCP is a broadcast on L2. By definition those are not forwarded by the router. Your issue is the switch.

Break the L2 connection and have the router forward all your data packets between the networks. Subnet it out.