Number one use an absorbent to try and soak up any oil that you can. Hardware stores sell oil absorbents or you could use cat litter or saw dust. Sweep up the absorbent after a couple of days. Next use a heavy duty degreaser also can be bought at a hardware store and wash with a stiff brush and rinse with water.

If you don't have luck with that try a mixture of trisodium phosphate and warm water (follow the instructions on the container). Trisodium phosphate(TSP) can be found at any hardware store and usually comes in power form. Follow the same procedure as above scrub with a stiff brush and rinse bear in mind that you will have a clean spot where you scrubbed with the TSP and or degreaser. Do not scrub with a metal brush.

Another more aggressive chemical is oven cleaner. Spray it on, and allow it to sit for half an hour before hosing off.

You may also have to face the reality that the paving stones are permanently stained and flipping them over or replacing them may be the only alternative.

If you do have success removing the stain, consider sealing the paving stones with a concrete sealer. Your local hardware store would be able to help you chose the correct type and help with instructions. You could even get creative and add color to the sealer and change the color of the stones if you do not have success removing the oil.

Precautions:
Always use the appropriate safety equipment and follow label instructions. (wear work clothes and protective equipment, gloves, etc.)