How do you make a spring stiffer?

I think you are talking about adjusting the preload. I don't know if you can do it on the XR100 but I have done it on a TTR125. Look at the shock body, if there is a threaded section, it can be adjusted but losening the ring farthest from the spring and tightening the other toward the spring, then jamming them back together. This is easier with the rear end off of the ground.

You'll need to buy a stiffer spring or different shock on the XR100.