/interface bridge filter
add action=drop chain=forward dst-mac-address=FF:FF:FF:FF:FF:FF/FF:FF:FF:FF:FF:FF dst-port=67 ip-protocol=udp mac-protocol=ip out-interface=ether2
add action=drop chain=forward dst-mac-address=FF:FF:FF:FF:FF:FF/FF:FF:FF:FF:FF:FF dst-port=67 ip-protocol=udp mac-protocol=ip out-interface=ether3
If you have more than one bridge you can add in the rules out-bridge=XXXXXX too !