How to block DHCP traffic in bridged Tinc VPN?

[Dennis Wichmann]

Hello tinc folks,

I´m new at Tinc networking, so if you only show me a link to a solution 
of my problem, I´ll be very happy.
My problem is, I use a Tinc bridged network with 5 Fritz!Boxes routers 
to connect my whole family together. But I can´t use a DHCP service in 
the Tinc VPN, because if someone use DHCP request on a fare away 
location, he probability get my gateway for internet traffic, although 
he has is own local DHCP service in his own router. I know that a briged 
network is not separated, but I only want to block the DHCP ports 67/68.
My question is: How can I block DHCP traffic in Tinc service? I tried to 
use IPTables in Fritz!Boxes, but it won´t work, because of the bridge 
service which connects the local LAN ports width the Tinc virtual port.

Thanks,
Dennis