My wife's iPhone has iMessage turned on as she communicates with some friends via this, where as I've got unlimited texts so ive no need to use iMessage unless I've got no signal which is extremely rare. So my iMessage is turned off, but when my wife's messages me it comes to me as a normal text not an imessage. So really all you should need to do is turn off iMessage in the settings(I know you've already done that). Not sure why you still only get them as an iMessage.
You could go onto apple website and ask on their online chat or just phone them.