How to disable the form for certain users?

If the link is copy pasted the form again is available to non permitted users.

What can be done to ensure security in this terms?

It seems like you have already tried the method for the form to open only by those with a special link.

Allow Jotform to Be Filled out Only by Those with a Special Link

You can also improvise protecting the form by setting a password. Please note that this can be bypassed by advanced users.

Check this thread to see how to password protect your form.

https://www.jotform.com/answers/1126241

Let us know if you have further questions.