Please guys, i really need this.

Thank you so much in advance !

try modify the hosts file.

Block access to websites on a Mac
Editing /etc/hosts

1) To edit the hosts file and add a new entry, open up your Terminal and type the following (You’ll be asked for your root password because this is a system file):
sudo nano /etc/hosts
2) This will bring up a screen.To block any the sites, simply type it in. You can have as many sites map to 127.0.0.1 as you want:

3) Now save the file in nano by hitting control-o and the return key.

Note that you may need to run the following command for changes to take effect:
sudo dscacheutil -flushcache
This flushes your DNS cache.

@Benjamin: You are very irresponsible to tell anyone to use Terminal for something that can be done easily in the GUI.

@ROcketeer:
DO NOT get started with Terminal for this.
Click the "Go" menu, choose "Go to folder".
Type:
/etc/
and click the "Go" button.
Locate the file named simply "hosts". Drag a copy of it to the desktop. Drag another copy of it to some backup folder. If you don't have any place already for backups, make a new folder and call it "System-backups" or whatever.
Launch TextEdit. Use TextEdit to open the hosts file on the desktop, and add the line:
127.0.0.1 badsite.com
after the line with 127.0.0.1 localhost. It might look like this when you finish:

##
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting. Do not change this entry.
##
127.0.0.1 localhost
127.0.0.0 badsite.com
255.255.255.255 broadcasthost
::1 localhost

Save the file, and be sure it has no extension, just named "hosts". Drag it back into the "etc" window, and when you get the message, click "Authenticate", click "Replace", and type your admin password. Restart.

Parental Controls 10.5, 10,6 http://support.apple.com/kb/ht2900
10.7 http://docs.info.apple.com/article.html?path=Mac/10.7/en/mchlp2912.html

If you want to actually block a site, you could do many things to the macbook but they might get moved, suggest that you actually block the sites on the firewall in the router you use to connect, or better still there is a dns service called opendns its real, you can sign up for it and set up rules as to sites and types of sites. then all you do is either change the settings in the router or the settings on the macbook to use the service. It blocks a great deal of things or specific sites, i use it to protect the kids in our house.