Like so many others, we've been there......only we did have a package at the time for 500 messages per month, only he used 7,000. I opened that rather fat envelope and know my neighbors heard me scream "Holy Mother of God!"
I called Cingular and I know they thought I was the mother from hell. I didn't understand how or why ANYONE could text that much. I just knew there had to be some big mistake! They did adjust my bill retroactively giving us the extended package - Thank God! The rep explained that people will text instead of calling so as not to use up their minutes.
I don't think you're being too harsh and when his privilege is restored he won't abuse it again, I'm sure! We now make our son pay for the extra service.
Definitely call and see about getting the bill reduced, they will usually work with you. Afterall they do want to keep your as a customer!