I don't know how you would do it on a per-assignment basis, but I have an idea for how you could do it for the whole course.
You could define a role that is just like your regular students, but cannot see the gradebook -- and assign that role to the students that you wish to prevent from seeing their scores.
Hi Manoj,
This is possible for Moodle 2.2. First you need to create a new student role as L C already mentioned (Creating custom roles). In Assignment administration, you are able to select individual students and give them local roles.
With kind regards,
Hartmut
