Sorry to hear that you are getting SPAMMED like that @Rikk1960, Initially when I read the heading of your post I was thinking the whole public registration angle. Reading further I see that you have have dismissed that notion and I understand your frustration because I have been there.
It has been my experience that SPAM has changed for the worse. I don't really have a great answer for how to reduce SPAM but I will say that I have gotten the most SPAM from domains that have been previously registered. I don't know about your domain specifically but I do know that when you buy a domain you additionally get the baggage from that domain. I have myself purchased domains with some unscrupulous history and it does take time to rectify those issues. I actually don't even advise that one should try to reform a domain.
I hope your issue isn't one of history. That's all I can think of because I haven't known a fresh domain to have a more than normal amount of SPAM. Unfortunately laws on this are behind technology and I find that going back and forth sometimes means more SPAM. I wish I had something more I could suggest.
...turns out that my two cents is worth less or more depending on the current exchange rate.
roy darling *my posts seem a lot shorter in my head