+1 - the right way is to get the right rims
Changing the spring stiffness just means it wont hit as often, ie you have to hit a bigger pothole/dip whatever to make it touch. Its not a fix.
Its illegal, with good reason, to have the rim/tyre touch any part of the car, other than the mounting of course. You might be able to do something with spacers but i don't know if they're legal.