Thanks for writing. Some types of passionflowers spread via runners underneath the soil; the best way to prevent them from coming up in your grass is to install edging that's about 6 to 8 inches deep. That will create a barrier that prevents the roots from going into your lawn.
Other types of passionflowers spread via seed. If that's the case, then you'll want to deadhead your passionflower to keep it from forming fruits.
---Justin, Senior Garden Editor, BHG.com