Once you assign an activity to your students, you have the option to lock and unlock the assignment at any point. 

On the assignment detail page, you will see this toggle in the top left corner: 

Unlocked: An unlocked assignment means students are able to start new work or access it if they started it and did not submit it. 

Locked: If you choose to lock the assignment, students will not be able to start a new submission or do additional work on existing submissions. A locked submission will look like this:

Use this feature to ensure that your students are doing their work when and where they should be.