You can block specific sites by modifying the Mac's /etc/hosts file.
The easiest way to do that is to download a copy of the free TextWrangler:
http://www.barebones.com/products/textwrangler/download.html
Once you have done that and installed TextWrangler, click on the desktop to make sure you are in Finder. Then pull down the GO menu to Go To Folder. Copy and past the following line into the Go To Folder box and hit return:
/etc
This will open up a Finder window with the hidden /etc folder.
Look for a file called "hosts". Right-click on it and choose "Open With" and choose "Other", then "Text Wrangler". This should open the file in TextWrangler.
Your default hosts file should look like this:
##
# Host Database
#
#
# localhost is used to configure the lookback interface
# when the system is booting. Do not change this entry.
##
127.0.0.1 localhost
255.255.255.255 broadcasthost
::1 localhost
fe80::1%lo0 localhost
You can then block any URL by adding some lines follow the line "255.255.255.255 broadcast host"
For instance, if you wanted to block YouTube, Facebook, you would add the lines:
0.0.0.0 youtube.com
0.0.0.0 facebook.com
so your hosts file would then look like:
##
# Host Database
#
#
# localhost is used to configure the lookback interface
# when the system is booting. Do not change this entry.
##
127.0.0.1 localhost
255.255.255.255 broadcast host
0.0.0.0 youtube.com
0.0.0.0 facebook.com
::1 localhost
fe80::1%lo0 localhost
In other words, adding a line with 0.0.0.0 followed by the URL you want to block will block that URL.
Once you have done that, save the file and reboot your computer. You should find that you can no longer access the blocked URLS.
This can also be used to block ad sites like doubleclick.com
