You could change wireless interface ARP setting to ‘reply-only’, then set the DHCP server to add ARP entries for leases.
http://forum.mikrotik.com/t/add-arp-for-leases-what-does-it-do/79806/1
This will prevent wireless users from assigning their own IPs.
However, you might want to investigate how network_1 and network_2 are being bridged together [as that’s what the symptoms you’re reporting suggest to me].