Excluding other HTML markups

I would like to know if there is a way to prevent people from adding their own HTML code to textareas.
I mean that they can use the buttons of the toolbar and do what they want with CKEditor, but I don't want that them to add their own HTML tags.
Of course, I will remove the "source" button, but that's not enough, they can add what they want. Is there a way to make a difference between HTML code generated by CKEditor and HTML tags added manually ?
My test page is here : http://www.lgv.on-web.fr/valinfotest/testcke.php

Please forgive my english, it is sometimes different to explain something in a foreign language.