Does anyone know how to stop anybody that goes on my computer from accessing www.kazza.com
Or from accessing Yahoo messenger??

What kind of Operating System are you using.
The point being you can usually find from the help menu a way to assign a password to any user. Also you may want to disable the feature that is allowing you to use a secured website without it remembering your password, in other words you will have to enter it each time you visit the site and keep your password private.