SpamBoot are searching for "Powered by MYBB" so I only requested to delete that not the link to mybb.com
Yeah, JordanMussi offered one suggestion, make a small image with the Powered by MyBB text on it and use that in place of the actual text, or you could also do:
<a href="http://mybb.com/" class="poweredby"></a>
.poweredby:after {
content: "Powered by MyBB";
}