I keep seeing tasks I won't be using again in the list that appears when I click the plus sign to create a new task. How do I get rid of them?

The autocomplete task list is populated with recent and recurring tasks that meet one of the following conditions:

Recent: The task has been added to your day plan within the last seven days and not marked complete.
Recurring: The task has been added to your day plan and marked complete at least twice within the last seven days.

If the task you want to remove from the list classifies as recent, navigate to the last day on which you added the task, hover over it and you’ll see three glyphs appear to the right of the task’s associated project. Click the middle glyph—a checkmark—and the task in question will be marked complete. It will no longer appear in the pre-populated list of recent and frequently-used tasks. Note that if you’ve already finalized your time on this task, you’ll need to click the lock icon to open it for editing in order to mark it complete. Then simply click the lock again to re-finalize.

If the task you want to remove from the list classifies as recurring, you can either A) mark all but one of its instances in your day plan as incomplete and hit F5 to refresh Daycast and remove it from the autocomplete list, or B) wait until the seven day window has closed, when the item will drop off on its own.