If you're using IE, you can block individual websites from placing cookies on your PC, which in many cases will have the effect of blocking access to the website altogether (if it's a site like Bebo, which relies on cookies/logins):
To customize your privacy settings for an individual Web site
In Internet Explorer, on the Tools menu, click Internet Options.
Click the Privacy tab, and then click Sites.
In Address of Web site, type the complete address of the Web site for which you want to specify custom settings. For example, http://www.microsoft.com
To specify that you want Internet Explorer to always allow cookies from the specified Web site to be saved on your computer, click Allow. To specify that you want Internet Explorer to never allow cookies from the specified Web site to be saved on your computer, click Block.