[freestockoption]

How much do you care? How much do they care? I go to lots of people's houses and they openly give me the wifi password to their network.

That said, having security is good. And building networks is fun. :)

If you are comfortable with a little Linux you could get a router and reflash with OpenWRT. Divide into 2 different VLANs. Have one port on the router connect to your VLAN and the other port connect to your neighbor's. If you use your own router on this port, you would treat this port as WAN and your own network as LAN. It would probably be NATed so your neighbor would have a harder time getting into your network. The OpenWRT GUI makes it pretty easy, but it helps to know a little about subnets and routing.

For a commercial solution, you could get a Ubiquiti Unifi gateway with a Unifi switch. Basically does the same thing. I'm not sure I would say their GUI is easier or harder than OpenWRT. You still need to know a little about networking.

If you don't sufficiently trust your neighbor having access to the WAN port, you could connect your own VPN router to your port on the VLAN.