HOW CAN WE STOP SPAMMERS FROM TROLLING OUR COMMENTORS!

It has just come to my attention that a VERYoverly keen author who has been, shall I say overly soliciting people to review his book has JUST taken one of my subscribers emails and sent her his standard "review my book" emails?    She is not a member of book blogs, nor is she even a book blogger, and I was VERY upset with this person considering I JUST  sent him an email HOURS before telling him to STOP sending me the SAME EXACT email considering he and I previously had been in contact about his book, and he was going to send me a hard copy intead of ebook, blah blah blah.  And TWICE more AFTER we had emailed back and forth he sent me this original email introducing himself, asking me to review his book.. yadda, yadda.

SO

I then asked him to PLEASE stop emailing me the SAME email and so he wrote back said he was sorry and took me off of his newsletter list.  And then about TWO HOURS later I get an email from one of my faithful and lovely readers saying she got this email and I scroll down and YUP it's him. 

AAAARRRRGGG!!!!

So this incident then  got me thinking.  When most book bloggers hold contests we ask that people comment and leave their email address that are then published for the entire world to see.  Huh.  I'm not exactly sure how to rectify that, but after my Outlander one is done what I may do is somehow keep track of the entries and then not publish.  Or have people enter by emailing me privately.  I'm not sure, but this really bothers me, and I know it was unnerving to this person as well.  Then I think, well, maybe this is just one of those hazards to entering in a contest?  Blah. I dont' know.  AHHH!

Any thoughts???? Especially much more experienced bloggers out there????