There are two ways that you can remove assigned people from an Item.
Here's how you can remove them directly from the Item:
1. Go to your Item and select the contributors tab
All the people assigned to this Item will have a tick next to their name.
2. Uncheck the people you want to remove
Unchecking them will immediately un-assign them from the Item. No one will be notified of this change.
Here's how you can remove all assignees from multiple Items at once:
1. Go to your Content tab
2. Select Items and then select assign people action
3. Remove all assignees from selected items
Click on the link to remove all assignees from the selected items. No one will be notified of this change.