How do I tighten sway bar end links?

I am replacing the end links between the front sway bar and control arm with genuine Subaru end links. Unlike aftermarket links that have a hexagonal inner collar that can be used with a box wrench to prevent the shaft and inner ball from turning while the nut is being tightened, the Subaru end links appear to have only a smooth round washer. Is there a way to prevent the shaft from rotating without damaging the boot?