UPDATE: Beeper, through their Twitter account, is saying that they’re working on it and “hope to have good news to share soon”:
Work continues to fix the issue causing the Beeper Mini outage. We know how hard this has been for those who loved using Beeper Mini, and we’re extremely sorry for the inconvenience. We are feeling good, though, and hope to have good news to share soon.
You need to de-register your number.
Beeper already deregistered the numbers, but it takes 24 to 36 hours for Apple servers to forget the deregistered numbers.