You are able to delete players or exclude them from the exercise. If you exclude a player from a particular exercise, you have the option to include them later on when you run the exercise again. 

Follow these steps to exclude players:
1) In the 'Manage Teams' tab, click on the 'Arrange Teams' button 
2) Drag and drop players from the 'Teams" box into the 'Excluded' box - these players won't receive any content for the duration of the exercise
3) Click the 'Save' button

To delete a player permanently, you can do so by clicking on the X button in the 'Team' box.

Below is a video of how to exclude players from the exercise: