If you do not already have a hosts file, you might have to create it. The defalut file is hosts.sam, but you don't want to edit this file. If no file exists, you will have to create one called just hosts (not hosts.txt)
After following binoyxj steps above, you can then run the command nbtstat -R which will flush any cache and re-read the hosts file
This is a pretty neat little hack to block access to a site. Although if someone knows how the host file works it can be easily be bypassed.
I would suggest making the change at your router level (password protected) which would make it harder to get around. Some routers have a "parental controls" option that allows you to setup a block list. Or otherwise, you add a firewall rule to prevent packets from passing to the specified IP address.
The hosts file is a great suggestion, but like y'all said, it can be easily bypassed.
The IE option is good as well, and can be undone in a moment or two.
The router is definately the best way to go. Make sure and change the username and password of the router to something other than the default.