I must disable a particular category on my small WordPressblog. Allows refer to it as Foo. Then when a person attempts to enter www.blog.com/Foo, they get rerouted to my 404 page. How do i do that with htaccess?

That's one method to get it done.

RewriteEngine On
RewriteRule /Foo 404.html [R=404,L]

You might alter the slug for that Foo category not to be Foo any longer.