Try commenting out the server address in /etc/yp.conf
No route to host messages usually signify a routing problem, the remote network doesn't exist, the routes on the server are bad, the NIC cable is unplugged. If you want NIS to continue to work, try checking those things.
If you need to stop NIS altogether, then use the "service" commands.
Try www.linuxhomenetworking.com for more help on NIS.
Hope this helps.