If you are cleaning up your website outbound links, you might be looking at plugins which link back to their authors.
To remove the powered by, open the following file using an ftp explorer or other file explorer. This is a code hack so make sure you have a valid backup first!
Search for "powered"
You'll find the following line:
if ($this->config->get('credits', 1)) echo $this->poweredBy();
Comment it out completely and the link will be gone! Remember visit their site and donate or otherwise link to them!
//if ($this->config->get('credits', 1)) echo $this->poweredBy();