In Word 2007, the style automatically converts to a heading style from the
gallery every time I double space. How do I turn this off?
First, allow me to shake my finger at you and say "Don't 'double-space' by
pressing Enter twice!" If you want space after a paragraph, set it (or
preferably its style) to use the correct amount of Space After, and then press
Enter only once.
That said, you can turn off the automatic formatting by going to Office button >
Word Options > Proofing > AutoCorrect Options > AutoFormat As You Type and
unchecking the option to apply built-in heading styles.