While you can't cancel an active add-on, you can prevent it from automatically renewing at the end of your current billing period. The current add-on will continue until its expiration date, after which the system will automatically cancel it.
To cancel the add-on:
Log in to Rebrandly.
Click Usage.
Click Add-ons.
Click the Trash icon to remove the add-on.
This will cancel the add-on renewal.