It seems like your users are pasting HTML code, while you are using a "text-based" editor, e.g. DHTML.

You have to tell your users to stop posting HTML code, or you can select in Preferences an HTML-based Editor, such as TinyMCE.

If you continue to use a "text-based" editor, after the posting you can go to the article's Edit, and "un-check" the "disable HTML"

Please be aware that HTML code might create security issues, if users place malicious code.