As an Admin, you have the ability to lock sessions so that users cannot make edits to them. Prior to locking any sessions, you must enable the session-lock option within your event site's setup properties.
Enabling Session Locking
From the Admin portal, click on Event > Setup Properties:
Click on the Event Options tab:
Click the Edit details button in the top-right corner:
Under the General section towards the top of the page, make sure the box next to Allow admins to lock and unlock sessions from being edited is checked.
Once you Save your changes, you can pick a session to lock.
From the Admin portal, select Sessions > All Sessions:
From here, you should see a column with a lock towards the left hand side of the session grid. If you don't see this column, don't worry - you may just need to adjust your Session Summary View Settings. Clicking on any of them will lock or unlock that particular session:
Additionally, you can click on an individual session via the Session Details icon and lock it from the top-right corner of the page:
As always, be sure to hit Save to save all your changes.