Wow, this has been going on for at least a year. I can't believe that Apple hasn't already fixed this problem, it really calls into question their commitment to doing the right thing.
In the hopes of actually solving your issue, I hate to suggest this, but try reconnecting your number to imessage (and make sure you get any new imessage texts), then properly turn off imessage from that device. The defacto unplug it plug it back in.