since you say that a hitch (trunk)
rack isn't possible (understandable)...
Can you fit your bicycle inside the Prius?
Or in the trunk? I imagine that if
you don't have the quick-release locks
then maybe taking a wheel off isn't
going to be the best/easiest solution.
Otherwise, there are roof racks that
you can buy. Yakima and Thule both
list the Prius in their online catalogs.
Probably a little more expensive than
placing the bike in the car, but...
it's another option to look into.
I have seen a couple of Prius owners around the Bay Area with bikes on roof racks. That would seem to fit with the other respondent who says Thule lists Prius in the catalog. I have the folding Dahon bikes which fit in the trunk so don't have personal experience with the racks, but they looked secure.
I carry bikes on a Rhode Gear Speedster Shuttle 3, which straps onto the trunk. I think it would have more ground clearance than a receiver hitch, and it is cheaper than my Thule roof rack (which I use for camping gear and building materials).