18V - be very careful, I think that's completely the wrong sort of power supply for this setup...
If the stepper coil resistance is 0.8 ohms as stated, this will draw a current of 18/0.8=over 20 amps! According to Ohm's law (and give or take a bit for losses in the driver itself).
This stepper will draw 2.8 amps from a 2.25V supply, in theory. (Again from Ohm's law)
Really, this stepper isn't made for driving from a fixed voltage driver as we amateurs use. It's built for running from a proper current controlled driver ("Chopper" driver) as rgbric said above. These things are designed to push 2.8 Amps (say) into the driver at whatever voltage is needed - this also gets decent high speed performance from the stepper. (Normally a stepper will not be able to turn as quickly if driven from a simple constant voltage.)
(Look at the fancy drivers recommended for this, on the page you linked to.)
The stepper you have is decent, but to drive it from a simple driver like an L298 you should probably use a much lower voltage power supply (a bit over 2.25V - say 4 volts, to overcome losses in the L298 itself).
Alternatively if you have to use a higher voltage, you could use a resistor in series with each of the two windings, again as said above. It would need to be a power resistor though, because it will get hot! Also it will be very inefficient - all that energy is wasted as heat in the resistors. I think for 18V at 1A per phase you will need two 15-16 ohm (or higher for less current) Power Resistors rated at least 15 Watts... Calculating all that needs a bit of Ohms law work again.
Finally the advanced option is to use the raspi as a Chopper driver directly - don't just switch "on" the stepper, instead drive its coils with PWM signals so they are "on" only for about 10-20% of the time. This is an advanced topic though, one software glitch and you could turn it on fully and everything would overload immediately again.
I think, unless you can suddenly get a 4V (ish!) power supply at a few amps, the two power resistors trick is the quickest way to get you running. Be very careful with that 18V though, easy to fry things. (I fried a raspi once with an 18V unregulated supply I made!)
Disclaimer - I have never driven a stepper motor this powerful, just giving you the theory, and I've never tinkered with the proper controllers. (I've seen them running though - very nice industrial stuff and worth every penny.)
ps sorry about the length. I tried to shorten this, I really did!