i am taking care of computer lab having 30 computer in it .all connected in lan with server.
is it possible for me to switch off network or say internet for any single computer.(any software or anything)
Go to the router, disconnect the computer connection. You can also set access by MAC address. For only blocking internet access but not LAN, you need to use a router that has better controls than most home routers would have, as in a home router you can lock internet access, but for the whole device, not a PC at a time. You can also use the firewall and gateway to control who has internet access through PC IP and through the server using user group membership and computer AD membership.