With the quicktag_settings
filter:
function wpa_47010( $qtInit ) {
$qtInit['buttons'] = 'strong,em,link,block,del,img,ul,ol,li,code,more,spell,close,fullscreen';
return $qtInit;
}
add_filter('quicktags_settings', 'wpa_47010');
The default is:
$qtInit['buttons'] = 'strong,em,link,block,del,ins,img,ul,ol,li,code,more,spell,close';
Though 'fullscreen'
usually gets added too at the end. So I just deleted the 'ins'
button.
Edit to add:
If you wish to create a custom button the following tutorial might help.