You can only lock the checkpoints in the older assessment module, not in the newer Simple Assessments area.
In the original assessment module you can lock the assessment Checkpoints by going to Admin > Config > Checkpoints Lock.
This enables you to set a date when the option to enter assessment data into a particular Checkpoint will cease. You will be able to view the data entered at all Checkpoints in the markbook, but if the Checkpoint is locked you will be unable to enter or change any data.
To enter the dates, click on Edit, then the 
to select a date from the pop up calendar.
Click on Update to save. On the selected dates each Checkpoint will no longer be available to alter data, but you can edit the dates here if this is required.
NOTE - to set only one checkpoint as open, all others must have a lock date before the current checkpoint's lock date. When the current checkpoint reaches its lock date, the next one to be opened should have its lock date edited to a future date.