Can I have them towed?

I have Bible-thumpers in my neighborhood and they've decided to park their car under the shade tree in my yard instead of the parking spaces that are right across the street (the parking spaces aren't shaded). I don't see them roaming the neighborhood so there isn't anyone for me to confront so I need to know what I can do. I tried calling my police dept's non-emergency phone number but I was routed to the county sheriff's office. Can I call a towing company and have them tow these people's van away at the owner's expense? Please help!