You can remove bbc buttons in shoutbox settings (Disable BBCode:). You have to remove the sound button in template -
Shoutbox.template.php
/// disable sound
echo '
<!--img id="shoutbox_nosound" onclick="Shoutbox_SetStyle(\'nosound\');" onmouseover="Shoutbox_Hover(this, true)" onmouseout="Shoutbox_Hover(this, false)" src="' . $settings['default_images_url'] . '/bbc/nosound.gif" alt="nosound" style="background-image:url(\'' . $settings['images_url'] . '/bbc/bbc_bg.gif\');cursor:pointer" /-->';
Don't know if this will solve your problem, input field and send button slide right when buttons are removed.