If you format your Wii's internal memory, you will see that your ID changes as well. Although, I don't see why sharing your ID is a bannable offense. That is what Nintendo wants you to do. That is why they give it to you.
Your Mii Code is not directly tied to whatever Nintendo uses to identify individual consoles. They probably use MAC Address or some other means.
I guess they could build a database of all the users MAC Addresses, Mii IDs, and then
troll{cough} forums looking for people who not only post their ID, but also make mention that they may have a modified console (which that person could be lying about, to be "cool") Then go check it out and verify if it is all true. Then ban that single user's MAC Address. all the while assuming they don't spoof it. Doesn't seem worthwhile to me. And, a bit too risky. I could easily sabotage any console I had access to, if that was the case.
Please do not post questions or comments in the
Wii Mii Directory Pin, only your ID if you want to be added.
Well, if your ID changes when you format I guess there's little they can do. And, like you said it would be a waste of their time and resources.