Since there is no air preload on (what I presume is) that Suntour fork, you are stuck with getting a heavier spring for it or living with it.

Spring preload does just as its name implies: it preloads the spring to be a bit stiiffer in its initial part of the stroke, lessening initial suspension sag. It does nothing to increase the spring's manufactured spring rate. I have to ask - - are you actually bottoming the suspension out? Or does the 'squishiness' of the fork just bother you? If it's the latter, give yourself some time on it. If you are actually bottoming the fork out, go back to your sbike shop, tell them the problem and see if they can order you a stiffer spring for that fork.