The L298 chip is just an H bridge without any kind of current limit control. Look better for some kind of chopper driver like this:
http://www.electrofunltd.com/2012/08/elfdrv1-stepper-motor-driver-coming-soon.html
or something based on a TB6560 or similar.
They even could be cheaper that your L298 based board and will allow you to keep the 18V voltage (obtaining the maximum torque of the motor). If you put a resistance in serie, it will need to be really really big (and expensive, therefore) and it will be a waste of energy ... In this way your system will be more a stove than a motor system 8)