It is not possible to stop the IP Manager from creating the NetworkConnection relationship.
A network connection is a connection between two interfaces. A network connection can be a logical connection or a physical connection. An example of a logical connection is when routers are connected using a virtual circuit and none of the intermediate network devices are included in the topology. An example of a physical connection is when routers are connected using a serial or point-to-point connection.
In this case, the Internet Service Provider places the ADSL routers in the same subnet. IP Manager does not discover routers in the Internet Service Provider cloud. When it discovers the ADSL routers which have no relationship, the NetworkConnection and NeighboringSystem relationships are created.