You can disable HTML links for all guests. That's the best way to do it. If it's a guest you trust, then he should become a member so you can allow HTML links.
Follow this:
ACP>Configuration>Icy Phoenix>Posting and messages>
Scroll down to disable the html for guests via the radio button see pic:
You can also disable globally HTML, and make guests read only by going to:
ACP>Configuration>Site>
2 things;
Under the heading on the right (options on that page)
User and Forum Basic Settings; disable HTML
Guest allowed; read only