to keep from doing that, just keep one foot on the brake about half applied so the SECOND you let go of the throttle, the car stops. it is pretty easy to feel the bump on the end of the ramps if you know what you're doing. cinderblocks are not neccisary if on level ground because the tires sit in a little dipped in deal at the top and plus the car is in park so it cant roll anyways.
also. if you have ground effects and the ramps will not clear them, all you have to do is take a couple pieces of wood, lets say 2x4s preferribly 2x6s under each tire and then line them up along the slope of the ramp so the car just rolls right up on to the slope of the ramp without skipping any gaps. and with enough room, you can really clear the lowest ground effects if done correctly.
my ramps are definitely the best fluid changing accessory i know of, so much easier than jack and stands, MUCH safer and FASTER.