If you've made a mistake in assigning material to a learner's to do list, you can remove via their individual report.
To do this, select 'Reports' in the top left hand corner menu:
You will need to enter the learner's name in the search bar, and you will need to at least specify the title. You can use the other dropdowns to filter further within the title if desired;
Click on the learner's name:
Select the tab 'To Do'. You can then select 'Remove test' beside any test name that you want to remove from the learner's to-do list.