1. 
2. Click the 'Permissions' link in the left-hand panel under the heading 'Security'. This will display the space's current permissions assigned to the different groups and users.
3. Click 'Edit permissions'. This will bring up a new screen. See screenshot.
4. Uncheck the permissions you which to revoke.
   Removing the 'View' permission for a user or group will remove all access to that space for the user or group.
5. Click 'Save All' to apply the permissions.

To remove all permissions for a user or group, just remove the 'View' permission. The user or group will disappear from the list when you save the permission updates. There's no need to untick all the permissions for that user or group.