In the interest of offering a balancing opinion, "don't attribute to malice what can be explained with simple stupidity".
Sure, this could be a very shady recruiter trying to pollute your recruitment pool. But it could just as well be the equivalent of a sales-person at a consultancy company cold-calling potential clients, or Suzy Workseeker sending her resume to every company she can think of to see if anyone takes the bite.
The recruiter may not even understand the issues his actions are raising. Far from being unethical, he/she may just be ignorant and/or inexperienced (a state of being we all exist in at least once in our lives).
But, if you really are set on hiring this person, then you may be in trouble but that is all depending on the jurisdiction you are in and what contracts and agreements are set up. The main sticking point here is what type of agreement exist between the recruiter and the candidate. The standard agreement I've seen prevents the candidate from directly accepting work from a company that the recruiter has been in contact with on his or her behalf (i.e. your recruitment is "polluted" since you have no choice but to go through the recruiter to get this person).
So, the question then becomes: "have the recruiter been in contact with you on the candidates behalf?" Well, that's the kind of question that keeps the world full of lawyers. A number of factors can play in here and the legal determination differs on where you are. Between the candidate and the recruiter exists a good-faith assumption that the recruiter will make his best effort to satisfy his side of the bargain. Unsolicited inquiries may be far away enough to void their contract. It could also be in this case that the candidate can accept a direct approach and defer any claim from the recruiter onto you. And since the initial contact was not only unsolicited, it was expressively unwanted, the claim falls.
Bottom line: I'm just a random guy on the internet without any legal training or knowledge of laws and regulations where you are. If you're really set on hiring this person, unfortunately it sounds like you need to talk to a lawyer. It's a sad state, but there you are.
If you're not intending to hire this person, you could contact the recruiter directly, explain what his email caused and if it ever happens again, you will refrain from ever using their services, advice all your contacts to do the same and report them to whatever equivalence of "better business bureau" exists in your area. You could also contact the candidate and explain what has happened and why you are now unable to hire him/her regardless if you wanted to or not.