I'm trying not to use it, but Apple won't let me.
I was responding to the person I quoted. Your problem is not that Apple won't let you use the service - you already can't use the service because you're not using an iPhone. The problem is that your friends' devices are still sending messages to you via iMessage.
RTWG's scenario sounds plausible, and the more I think about it (based on my experiences), the more it seems like the issue is something on your end. Again, ensure that any other Apple devices that you have (Mac, iPad, iPod) are not set to receive iMessage messages through your phone number. To be safe, I would completely disable iMessage on those devices - at least for now. If you're using OS X 10.8.2, open Messages and do this through the preferences - your Mac can still receive iMessage messages even if the Messages application isn't open.
The reasoning behind that is based on how iMessage's switch-over occurs. If the device sends a message and doesn't receive a delivery confirmation after a while, it will send the message again via SMS. This switch-over isn't always perfect, but after a few hours (or certainly a few messages) it will happen. In my experience, the text messaging button turns green and remains green for that contact
until one of their devices reconnects with the iMessage service. The fact that your friends' iPhones continue to use iMessage seemingly indicates that they're receiving delivery confirmations from one of your devices and/or that one of your devices is logging back in and alerting other Apple devices to the fact that you're iMessage-capable.
You can test this for yourself, as well. Have one of your friends send you a text message, and check to see if it says "delivered" underneath the message. If it is, you have a device that is receiving the messages and that keeps your friends' phones sending via iMessage.
Edit: I briefly read over the first page posts again, and saw you mention that you had disabled iMessage across all devices. Check it again, and also check to see if your friends' phones are showing the delivery confirmation when they send a text message. It's possible that your old phone is still receiving the messages and/or logging in through your account, but if you removed it from your devices in iCloud it should theoretically be cut off...