If you've been adding a calendar sync for one or many tasks lists you can remove it from your agenda app anytime.

Google Agenda

Open your Google Agenda page
From the left side panel, fly over the Azendoo tasks list webcal subscription you'd like to remove and click on the 3 dots icon:

Then choose Settings from the menu:

Scroll to the bottom of that page and click on UNSUBSCRIBE:

Confirm by clicking on REMOVE CALENDAR: