Admin CP > Styles & Templates > (Choose your style) > Main CSS > Additional CSS Definitions
Look for this:
HTML Code:
/* ***** styling for 'big' usernames on postbit etc. ***** */
.bigusername { font-size: 12pt; }
and replace with this:
HTML Code:
/* ***** styling for 'big' usernames on postbit etc. ***** */
.bigusername { font-size: 12pt; font-decoration: none; }