In the old days, yes, in the days before DHCP we had a similar problem on our cat 2 ethernet. I went over to the other guy and told him he was using my IP address. We went to the sysadmin and we agreed that he would use this range and I would use that. Done.
I suggest you talk to your neighbor. I suspect that they are clueless that they're messing with your network.
Consider this a lesson in design for robust networks and being neighborly.