You could create a separate router per network.

By the way, here is the routing table in my router:

$ ip netns
qrouter-3d1dcda5-c743-4535-bcbb-5a1c651adce1 (id: 1)
$ sudo ip netns exec qrouter-3d1dcda5-c743-4535-bcbb-5a1c651adce1 /bin/bash
# ip r
default via 192.168.1.1 dev qg-426ef0ad-56
10.100.100.0/24 dev qr-45b6322e-2a  proto kernel  scope link  src 10.100.100.1
10.200.200.0/24 dev qr-47b002a5-04  proto kernel  scope link  src 10.200.200.1
192.168.1.0/24 dev qg-426ef0ad-56  proto kernel  scope link  src 192.168.1.248

If you want a single router that doesn't route between the networks, I guess you would have to set up source policy routing, which Neutron's default L3 agent is probably unable to do. Perhaps other routers are more capable, for example Brocade's Vyatta plugin.