Replace HTML-Source in Output of wordpress?

Question

I'm wondering if it's possible to replace code in wordpress directly in the output (last instance before sending to the browsers). I use "the_post" and "the content" in my plugin: add_filter( 'the_content', 'myContent', 999);

But this version only changes the data directly in a post. I'm searching for a simple way for changing content after the html is ready for sending to the browser to make changes undepending from other plugins or the theme.

My current problem is, that I'ld like to set a category to a post group which should'nt be visible to the visitors (the other subcategories sould be visible). My idea was to replace the html directly in output.