If you have just 1 nickname with your account it will be just /ns drop nickname password or ie: /msg NickServ DROP Immortal mypass123
And if you have some other nicks grouped/linked to your main nickname/account first type:
/ns info accountnickname
You will get line like "Nicks : Immortal Immortal-away"
/ns ungroup Immortal-away
That will just drop the grouped immortal-away nickname.
So now we can drop our account/ main nickname with the drop command above.