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In addition to pruning your spirea shrub, you might also consider dividing it. After a few years, shrubs like spirea can become a little lackluster. Dividing them at the root is one way to revive them and remove old growth. You'll also get more plants by doing this. A cloudy overcast day is a good time for dividing plants as the roots dry out too much in sunny hot weather once they are exposed above ground. Give your plant a good watering the day before you plan to divide it. The day after watering, dig out the plant and remove as much soil as possible from the roots without damaging them. A hosepipe works well for this. To divide your plant: Using something like a spade or saw, cut the root crown into quarters. Each piece should have a healthy bit of root and above-ground growth. Remove any old root sections at the narrow tip and get rid of any damaged roots or any parts of the plant that seem rotten. Give the parts of the plant above ground a prune when you do this, removing any damaged or dead growth and some of the thicker oldest wood stems. Replant the root sections and you'll have new, vigorous growth when the plant re-emerges in the spring.
Consider dividing your spirea. Divide your spirea in the late fall when the plant is dormant. Divide your spirea. Give the roots a prune after you have divided them.