Clarity

Can we Disable certain tasks for certain resources?

You can not assign them to the task and then prevent them from adding unassigned tasks to a timesheet (global setting).

one user is looking for this functionality in Clarity, resource should be assigned to that task but should not be able to see it in his timesheets.

What you mean disable task - Did you mean open for time entry at the task level? 

one user is looking for this functionality in Clarity, resource should be assigned to that task but should not be able to see it in his timesheets. just checking is there any way to do this?

If you close the resource from time entry against the project (1)  or close the task for time entry (2) then they will not see it on a timesheet.

But;

(1) - affects the resource for all tasks on the project

(2) - affects all resources on that particular task

there is no control at assignment level (other than not assigning them in the first place.)

If we  have to do it through change request, for that we need some input that this is not configured or meant to be like this OR that we can do it via a certain way. please suggest.

Open for Time Entry isn't available on Assignments.  I raised it as an enhancement under the 'old' request regime but I don't have access to the 'new' enhancement request regime so I'm not sure if it's been raised.

If you have access, you should be able to see if it's on the backlog, or alternatively raise it as a new request

New Customer Innovation Process for Clarity PPM 

Have you considered using To Dos on a Task?  You could assign a To Do item to a person and that is not used for time tracking.

To Dos are available in Modern UX for Tasks.