start with your service provider.
How will I report for stolen or lost of my blackberry 8700c
If you were using an enterprise server connection, call your IT department and have them lock it as well.
Considering that this guy was posting inane cellphone questions (check the posting history) about the same time that some spamming reps from some celphone company were posting answers, I think he's part of a spam conspiracy.
Is this the great Spam Conspiracy of 1106 that was foretold by George Orwell?