you can DoS a single computer on a network through ARP table manipulations. This is the same (outdated) method used by systems meant to deny access to servers before sending them to a captive portal first and then once access is granted they'll modify the clients ARP cache to point at the correct server.
The same can be done to isolate a host from a network (deny him from the entire network). The simplest way this can be achieved is by associating the clients default gateway with the MAC address of a non existent computer. This isn't a flood, doesn't require an insane amount of bandwidth, just a couple of forged ARP packets targeted at the victim. Ettercap ships with an isolation plugin meant specifically for this.
If you wish to just deny him from a single website (instead of the entire network/internet) then you can perform a Man In The Middle Attack, write a custom filter/plug-in for ettercap that will drop all packets coming from a specific host trying to access a specific website.



