To close enrollment to new users in a series:
Go to your series page and on the left hand side (if you are on a computer) or towards the top (if you are on a mobile device), you will see several sections you can access.
Click on "Edit Series"
Under the "Toggle Enrollment" section you will be able to turn on and off the toggle depending on whether you are looking to open or close enrollment. Turn off the toggle to close enrollment.
Please note that you shouldn’t close enrollment to make a 1:1 series, unless your series is a part of a specific Schoolhouse program/initiative. Learn more here.