How can I ignore an incoming bluetooth connection/pairing request? Major Android Flaw
Unfortunately, I think it's not your phone, but the other device. When you tried to pair with it, it remembered that (I can almost tell for sure you did not only try, you succeeded to pair with it). So now, when the other device sees yours in range, it automatically fires up request to pair. That's how Bluetooth works, and that's why the protocol is so convenient.

Check if this thread works for you:
Ignore / stop incoming bluetooth connect/pair requests