If you're out in the boonies, and don't have a dork-nut on the stem, press the base of the stem (through the tire) against the rim, to stop it from pushing through.
And an aside from roadbuzz's encyclopedia of cycling lessons, volume 208, lesson #16,086,597:
Every so often, in the comfort of your home or garage, change a tire, or deflate, dismount, and remount a tube and tire using only your on-the-road bike tools. Especially if you use a frame pump. This way you can be sure everything is working when you have a roadside flat, and you'll have some sense what pressure is right, etc. (Be sure to check your patch kit from time to time, too. Vulcanizing glue dries up.)