Hey Marius, welcome to Three Community.
I'm sorry you're having trouble with cancelling an Add-on. The option to do that should be available in My3, when you log in using a browser. If you click "Your Account" then "View / manage Add-ons" do you see an option to cancel?
If not, I've sent you a PM with a few details on how to get in touch to get this sorted.
StephR
Mod tip! The author of a post can hit 'Accept as Solution', to highlight a reply that helped solved their query.
