You can never stop people who are wiling to die
May I use "that" instead of "who"?
‘That’ in your case is a pronoun which can replace the pronoun who.
Generally it is used as the subject or object of a relative clause, especialy one defining or restricting the antecedent, sometimes replaceable by who, whom, or which: the horse that he bought, the man that came, etc.
I have read and confirmed it in many places - both are okay though some believe that who-goes-with-people rule is still better. Nevertheless, that is a relative pronoun there so it'll also work.
Well, my personal choice is who as that (though serving as a relative pronoun) does not look used for a human/living thing as compared to who
Further reading here
Some people make a big deal out of it, but for instance in German relative pronouns "der", "die" and "das", which all correspond to the English "that", are used exclusively for this purpose. So it is not simply that ignorant people who do not know the difference use "that" instead of "who". I suspect that it has to do with the grammar of Germanic languages in general. Of course, you can use "who" to make the meaning clearer or to add emphasis. Using "that", however, is perfectly acceptable and, yes, it is correct.