"Spirea does respond well to severe pruning. The best time to do so is while it is dormant in late winter. But spring or early summer is the second best time. You can cut it back to within 4 to 6 inches of the ground.
It is puzzling to know what is causing the dieback. If it is a root problem, the cutback may force new growth, but you may see additional dieback unless something is done t correct the cause of the dieback."