Use your email program to classify his emails as "junk" and filter them out. Then you don't even have to see them unless you go out of your way to look for them.
This guy probably has many imaginary "relationships" and clearly doesn't need any encouragement to continue them.
If ignoring the emails still bugs you, change your email address.
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Other than setting up a way to block him, there is not much you can do to stop it if he is out of the country. If he were here you could probably get him arrested for aggravated harassment.
I am a former federal and State prosecutor and now handle criminal defense and personal injury/civil rights cases. Feel free to check out my web site and contact me at (212) 385-8015 or via email at Eric@RothsteinLawNY.com. The above answer is for informational purposes only and not meant as legal advice.