how to permanently remove an address from spam?

yes

thank you so much for your reply. the gmails are from different people with totally different subject

I may onetime marked one gmail as spam.

As Latitia pointed out, if the sender is in your address book, it should be a safe sender.  You may want to review that one Gmail block for any "bigger" rule, such as block from this domain.  That's a stretch but worth a shot.

can you please let me know how to do it? 

I don't use Cox webmail...if that's what you're using...but I could logon to find it.