Prune the spirea before bud break In spring, i.e. now would be perfect. Cut all the stems down to about 4-6 inches from the ground. You can use pruning shears and remove one stem at a time or hedge clippers and remove all the stems at once. Use sharp tools to make sure your pruning cuts are “clean” which facilitaes fast healing. You can also lightly prune spirea during the growing season to increase a second flush of bloom. Right after they finish their first bloom, lightly trim back all the stems. Again, you can use hedge shears or hand pruners for this job. Good luck and have fun!