The easiest way as far as I can tell is to turn off mobile data.
You'll get a popup message telling you that your (unintended) MMS will be sent when mobile data is back on. This means you've time to send your message as SMS and delete the MMS from the outbox before it's sent.
As an added bonus, you'll notice a significant improvement in battery life with mobile data off.
I'll certainly be more careful in future if I should get any more MMS images.
That's not the only source of MMS messages - in fact it's probably the least likely to result in an unexpected charge.
You're far more likely to generate an MMS by typing a message that's too long, being sent to too many recipients, using a file (e.g. a contact card) or by picking a contact's email address entry rather than their mobile number.