yeah i believe he's in a different state
Oh... so you got to him already, 'eh? Mwhahaha @_@ (the other meaning of "state") XD
I could already tell you have some seriously justified beef with this guy Eric so wanting to be insulting is rather natural. But there is no point wasting energy on this guy as that would be feeding the trolls and honestly, you might as well enjoy the time you have by doing other things that make you happy ("the best revenge is living well"). Also, doing malicious things to just infuriate people always leaves a hollow joy, unlike the fulfilling joy of actually making people happy. So chances are whether you do anything or not Eric will still feel that pathetic hollowness in his gut, which is a fitting punishment. That's my take on it as someone who could have become a massive jerk but thankfully knows better than that (being nice actually has a multitude of benefits).
I don't specifically think he can IP track you as I think the problem is that when you turn on Street Pass, your 3DS is basically connected to a public network. If you stop the connection, you stop anybody else from finding you. But if it's the case that he lives way too far away to Street Pass you, perhaps that wont do any good. But on the other hand, perhaps it was a real coincidence he found you again. You should definitely unfriend him as that feature could allow him to see whenever you are online. Next to that, if you have the same Nintendo Network ID I think your device would appear the same regardless of whether you are using a new one or not.
Unfortunately, I don't know that much about 3DS and online play. Changing your IP might be worth trying as some extra protection. From a quick Google search, there are two ways you can change your IP address. The first is to reset your modem as then it apparently reconnects with a different IP address. The second is to use Wi-Fi from somewhere else as a different place will result in a different IP address.
I hope this is all just a big coincidence and he really has no way of finding you. And while we're on that subject, I somewhat know what its like to meet a creep in the wild (but then I'm a woman, so that was inevitable
).