Messages on the mac can be automated with applescript.

    tell application "Messages"
      send "This is an iMessage" to buddy "[email protected]" of (service 1 whose service type is iMessage)
      send "This is an SMS" to buddy "+1234567890" of service "SMS"
    end tell

A robotic auto-dialer could fix that. Perhaps an app on a jailbroken phone could inject numbers into the input field?

It'd be easier to automate the Messages app on an OS X device.

Another concern would be that Apple is likely to deactivate an ID used to send this kind of malicious spam at any sort of scale.

> Another concern would be that Apple is likely to deactivate an ID used to send this kind of malicious spam at any sort of scale.

Apple IDs can be easily remade. ProductIDs can be faked on a Hackintosh.

The part you missed was: "at any sort of scale."

I'm pretty sure you would encounter ever-increasing levels of countermeasures as you tried more and more tricks at scale.

But users will just update to the latest version if a few days, and then the juvenile fun will be over.

There are a number of ways to automate text messages. That's how a lot of commercial text notifications work.