To delete Kanbanchi you need to simply remove it from your authorized apps.
Go to the list of the apps connected to your Google account
Your list will look similar to this, and you will find Kanbanchi on the list:
Click on Kanbanchi to open settings, then you will see a REMOVE button.
We are sorry to see you go. Please, be so kind to let us know why Kanbanchi didn’t meet your needs by dropping a message at firstname.lastname@example.org? Thank you for trying our app.