Until now, the only way to remove an item from the calendar that it's synced to was to delete the task/event/time block in Marvin. With this new feature, you can now unlink them from the calendar they're synced to while keeping them in Marvin.
There are two ways in which you can unlink a task from the calendar:
1) Shift-clicking on the calendar sync indicator
2) Shift-clicking on the task in the Agenda/Calendar view, and then clicking on the "Unlink with ... calendar" icon
Unlinking Events and Time Blocks
To unlink an event or a time block, click on it in the Agenda/Calendar view, and click on the "Unlink with ... calendar" icon