“no parking sign” installed attempting to stop residents parking on unadopted road

A “no parking access required” sign has been installed on a back garden fence which backs onto an unadopted road which forms part of Springbank Close. Part of the fence has recently been altered to become a gate large enough for the owner to get a vehicle through, which is parked in their back garden. Prior to this development residents of the unadopted road have parked parallel to said garden fence for years. The owner of this fence does not reside on the unadopted road and has front access to their residence, including a double drive and dropped kerb on the main public road adjacent. The area of land which their sign is trying to prevent people from parking is not within their boundary and is in fact within the boundary of a resident of said unadopted road. Are they able to enforce such a sign or can it be removed?