I think the problem most people are having is that they don't have access to the old phone, because it has either been sold, stolen or irreparably damaged. How do you switch off iMessages when you don't have an iPhone to switch it off on?
We went through this recently when my wife's phone was stolen. Aside from doing all the tricks above (especially unregistering it at the mysupportprofile website) you can also try logging in on a friend's iPhone, going into their iMessages settings (the "Receive at" section, as mentioned above), logging them out, logging in with your old Apple ID you used to use for iMessages and then switch it off.
Obviously your friend will have to switch iMessages back on again to reverse the process, but this seemed to do the trick with us.
If your friend happens to be on the same network as you, and you still have a micro-SIM card, you could also put that in their iPhone and try the same thing.
All I know is we were plagued with this issue (I had to switch iMessages off just to send my wife a text message on her replacement non-iPhone!) but after doing this, it sorted itself out.
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Edit: Ha ha, @Tempamental, you updated your post with the red text!
Message was edited by: drfrot