Hi Uday,
If the list of user fixed for ever that these two user will can do this, then you can hard code the user's name and write code accordingly in AT-SELECTION SCREEN OUTPUT. For that you have to use Loop at screen statement as mention by Bhaskar. Check the SCREEN table for attributes of input fields.
If in future the list of user, who can untick the check box, may change then it's better you use authority check.
IF the user's fall into a particular role, then you can create a authorization object and assign it to that role. Then you can check the authorization object in your program to achieve this functionality.
Regards,
Ajit