Blocking traffic between interfaces

[TmKBlackWolf]

Hi everyone,

I have three interfaces: LAN, OPT1 and VPN. LAN and OPT1 are bridged in order to make the firewall transparent. However, I want to make sure that traffic coming from the VPN interface can only be routed through OPT1.
I already managed to route all necessary traffic from OPT1 to the VPN tunnel, however I did not manage to block traffic coming from VPN to the LAN without blocking everything.
I have to deal with an already existing network and have to integrate this tunnel without messing with the existing architecture.

I hope you guys can help and I appreciate any advice you can provide