How can I prevent my device from automatically converting SMS to MMS?
You can't do so with your samsung official messaging application, all of what you can get is a pop notification that you are now in the MMS zone (Converting to multimedia message...).
I would recommend Handcent SMS application, available at the market, and has an option for not converting texts (SMS) into MMS
This answer should be updated: It is possible to prevent this with the offical messaging application. See these two answers. It seems that either restricted or free will work. See this question for an explanation of the creation mode setting.