One easy thing you could do is buy one of those SoHo routers (perhaps you have a spare one already?) that does NAT, DHCP, etc... They go for about $100 USD, I would guess maybe 80 pounds sterling (where the heck is my pounds sign?). You could plug that into your network and give it a WAN IP from your network (such as 192.168.0.5), then configure the LAN on the backend of that router to be some different subnet (such as 172.16.0.0/24). Of course, all that does is block the broadcast traffic from your Samba machines, and blocks any other type of "chatter" going *to* her machines, but she can still access things freely (would have to know where to look tho, because Windows networking will be masked).
The other option would be to put an extra NIC in your Linux router and give it another subnet (172.16.0.0/24). You could hook a switch up to that NIC and use that for the landlady. You can then use iptables and prevent any traffic on that NIC from going to your other internal NIC. i.e. from the landlady NIC, traffic can only go OUT, not "sideways".