You have enabled broadcasting to let other computers discover you on the local network.
There are two ways to disable the ‘Advertize on the network’ feature. Uncheck the box in Fig 1 in the tutorial here:
Or you can disable it in the server.cfg file on the server host:
# Enable or disable broadcasting the required information to let
# other computers discover this host on the local network.
# 1: Enabled. Other computers on the local network can find
# this host machine.
# 0: Disabled. This computer cannot be found on the local
# network but it’s still reachable by providing its IP
# or hostname.
A restart will be required for it to take effect.