How can I remove the "Powered by" link from the remote help desk?

To remove the "Powered by" links you need to purchase a HESK license.

The license can be purchased for a one-time fee which is not included in the recurring monthly fee because, once purchased, it may be transferred to any server.
 
You can purchase a HESK license here:
https://www.hesk.com/buy.php

After purchasing you can install your license from the Client manager:
https://www.hesk.com/client.php

If you need any help with purchasing or activating the license please submit a support ticket.