You can delete students individually from the roster. Start by going to Rooms in the header, then find the room you need and then click the roster menu. Look for the student that you want to remove and click the delete icon to the far right of their name.
A confirmation message will show. Click Yes to confirm that you'd like to delete the student account. Keep in mind that this is a permanent action and cannot be undone. If the student is already connected to Socrative and they're deleted from the roster, they'll be logged out and taken back to the student login page. To add the student again, you can manually add the student to the roster.
Whether you're trying to add or edit a roster, if you have an activity running in the room, you won't be able to add/edit a roster. You'll see the error message below:
To finish an activity, go to the Results tab in your account and click the Finish button. Check out this guide to help you on how to end an activity.