yep you're right... seems like that allow rule that I've configured in my firewall is useless
; however there's a new way of blocking neighbors directly at the /ip neighbors discovery-interface, using interface-lists, the steps are first add a new list named as you want (deny-mndp, it's my example); then you'll need to add interfaces at that list and finally you can set up a discovery rule by matching that interface list.
The commands will be like:
/interface list add name=deny-mndp
/interface list member add interface=bridge list=deny-mndp
/ip neighbor discovery-settings set discover-interface-list=!deny-mndp