I'm not sure you can prevent this. eBay seems to auto-change items to BO randomly. I don't know how to stop it other than monitoring your items and manually changing them back when eBay does the sneaky switch.
HOWEVER................
You can use eBay's mania for "Best Offers" to your advantage, whether you list with auction or BIN. In the Selling Details section of the listing form click on the Best Offer click box. Just below that you will see two click boxes marked "Automatically accept offers of at least" and "Automatically decline offers lower than". Click both these boxes and enter your lowest acceptable price for each. If a buyer makes a lowball offer, it gets automatically rejected and you will never get a notice or email. If the buyer makes an offer equal or higher than your lowest price, it will get accepted and the item will close the same way as if the buyer did a BIN. And you don't have to be bothered with all the back-and-forth offers and eBay can't sneak in a BO without your knowledge. You might even get lucky as has happened to me now and then and the BO will be above your lowest price. I list auctions only but it works fine for me. It still can't stop nagging emails from problematic buyers, but that's what your BBL is for.
