It happened with me too...
I sent 30 such MMSs by mistake (they were supposed to be SMSs) over that I was charged Rs. 2(INR) per MMS...
after a little R&D from my end I found that there is a setting that probably restricts sending MMS
1. Go to Messaging
2. Click on Menu
3. Then Settings
4. Scroll Down to Multimedia Message (MMS) settings
5. There is option called Creation Mode, select that...
6. It shows 3 options Restricted, Warning and Free
7. Select Restricted to Completely Stop MMSs
8. (Do at your own risk) I also Deleted the MMS-APN provided by my carrier Vodafone, you can find that in your Home -> Menu -> Settings -> Wireless and Network -> Mobile Networks -> Access Point Names
There would be 2 APNs, delete the one which states MMS (In my case it was portalmms)
I have also Noticed this MMS option is enabled automatically when the SMS is pretty long (crosses certain word limit)...
I hope so this Helps