I put my ToDoIst taks into Google Calendar and now prefer to just see them in ToDoIst - is there any way to remove them from the calendar once they are posted there?
Yes, it's possible to unsubscribe from the iCal feed. When you open your calendar, on the left side, you'll see two "categories": "My calendars" and "Other calendars" - each with a small arrow on the right side of the category name.
Now, click on the arrow next to "My calendars" and choose "Settings" from the drop-down menu. On this page (at the bottom), you will find the Todoist calendar and on the right side, you can choose "Unsubscribe" to remove it from your Google calendar.