If you are happy simply hiding them from your users, you can do this in CSS in a custom theme:
.qa-nav-main-unanswerered { display: none }
If you want to take away the links entirely, this can be done in a PHP custom theme using the instructions here:
http://question2answer.org/advanced.php#theme-advanced
I have found that you can easily remove a menu item by doing something like this early in your custom theme:
unset($this->content['navigation']['main']['admin']);
This obviously zaps the admin link, but I think 'unanswered', 'tag' or 'user' will probably do what you need.