When creating a hyperlink with BIRT, it's always underlined.  Sometimes you don't want to include an underline.  One way to get around this is to create your own hyperlink in a text control using href.  If you were going to link to google.com, you'd simply put the following inside a text element of type HTML:

<a style="text-decoration: none;" href="http://www.google.com">hyperlinkText</a>

The part that removes the underline is the "text-decoration" being set to none.  If linking to another report, you could just build your link with the dynamic text tag within the text control and do the same as above.