Q. How do I block a particular domain or keep my children away from undesirable websites?

The first step is always to create and Administrative login account on the computer, which the children do not have the password for.  Change the login account(s) the children use to Standard or limited privileges to prevent them from installing software or changing the settings.

For Mac, you can enable Parental Controls, Automatic filtering, and then customize it by entering specific sites in the Always Allow and Never Allow lists http://support.apple.com/kb/HT2900

To block specific websites from your machine you can use the Hosts file. Where you find the hosts file depends on your version of Windows.

Windows XP, Vista and Windows 7
C:\WINDOWS\system32\drivers\etc\hosts
Windows 2000
C:\WINNT\system32\drivers\etc\hosts

Before you start, backup the Hosts file and put the copy somewhere safe, in case something goes wrong you can revert to the backup file.

• Open the hosts file using Notepad or another text editor.
• Find the line that has 127.0.0.1 localhost
• To block website facebook.com, for example add these lines under 127.0.0.1 localhost:
  127.0.0.1 facebook.com
  127.0.0.1 www.facebook.com
• Save the Hosts file and and close it
• Now reboot your machine to apply the changes.
• Try to open www.facebook.com into a browser.

http://www.overclock.net/faqs/44143-how-use-hosts-file-block-websites.html

http://www.webpronews.com/topnews/2003/05/13/use-the-hosts-file-to-block-web-sites

In Internet Explorer:  Tools > Internet options > Privacy > Sites  then add the address of the site or the URL, and choose Block  (this only works for IE of course)

or

• Open Internet Explorer and click on Tools
• Click Internet Options
• Click the Content tab
• Click the Enable button
• Click the Approved Sites tab
• Type the website name you want to block in the box, for example, www.facebook.com
• Click Never and then OK
• In the next window, type and confirm your new password
• Click on the General tab and make sure to select Users can see websites that have no ratings
• Click OK

How do I prevent my children from accessing bad websites?
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Google for Parental Controls
http://en.wikipedia.org/wiki/Content-control_software

Some ISPs offer parental control software with your subscription – ask your ISP what is available.

Commercial products (not an endorsement)
NetNanny  http://www.netnanny.com/products/netnanny
Cybersitter http://www.cybersitter.com/predl.htm
HideTools http://www.hidetools.com/

Both Internet Explorer and Firefox offer content restriction
FoxFilter https://addons.mozilla.org/en-US/firefox/addon/4351

Content Advisor for IE
http://www.microsoft.com/windows/ie/ie6/using/howto/security/contentadv/config.mspx
http://windows.microsoft.com/en-US/windows7/Internet-Explorer-Content-Advisor-frequently-asked-questions

Vista and OSX have some parental control features built in
OSX 10.5 http://www.apple.com/macosx/features/parentalcontrols.html
Vista: http://www.microsoft.com/windows/windows-vista/features/parental-controls.aspx
Windows 7 Parental Controls and Windows Live Family Safety http://windows.microsoft.com/en-US/windows7/products/features/parental-controls

Microsoft also offers options to restrict the Windows features available
SteadyState: http://www.microsoft.com/windows/products/winfamily/sharedaccess/default.mspx

Spyware Blaster blocks the URLs of known malware sites from your browser http://www.pcworld.com/downloads/file/fid,23106/description.html

McAfee Site Advisor flags websites security risks as you browse: http://www.pcworld.com/downloads/file/fid,62594-order,1-page,1/description.html

Block Pop ups
Firefox http://support.mozilla.com/en-US/kb/Pop-up%20blocker
IE http://support.microsoft.com/kb/843016
http://download.cnet.com/windows/popup-blocker-software/
Safari http://www.technipages.com/safari-enabledisable-pop-up-blocker.html

General web browser security