Locate the form-anonymous.php file within templates/default/bbpress and remove the following paragraph.
<p>
<label for=”bbp_anonymous_website”><?php _e( ‘Website:’, ‘bbpress’ ); ?></label><br />
<input type=”text” id=”bbp_anonymous_website” value=”<?php bbp_author_url(); ?>” tabindex=”<?php bbp_tab_index(); ?>” size=”40″ name=”bbp_anonymous_website” />
</p>
The website field should now disappear 