To Hide A User
Find the user on the roster and click the chevron next to their name to bring up the menu and select Hide Row.
The user will no longer show up in your grade book view or in reports run on the grade book.
To Show a User
Under Manage in your Grade Center select Row Visibility
The next page will show you all users (including hidden). To show a students that were hidden in your grade view check the box next to their name and click Show Rows at the top of the page. Then Submit when all students desired have been shown, if you do not submit the changes will not be permanent.
The student will now show in your Grade Center and all grades they have earned will be displayed.