Not able to block a spammer’s email

I am suddenly receiving spamming emails from an unused email address I was assigned years ago, related to a job I no longer occupy.  When I try to block it I am told it is already in my safe senders list.  Went to safe senders list:  there are no addresses visible in the list.  I tried adding that address (it showed up) and then deleting it (it disappeared), but on the Blocked Senders page I got the same message that that address could not be blocked because it is on my safe senders list.  If so, it's invisible/unreachable.  I am receiving many, many emails per day from this spammer.  Can anyone tell me what I can do about this?  Thanks!