The good news is that spireas can be cut back severely, even just above ground level, and they rebound. Cut branches at a 45-degree angle with clean pruning shears or a saw if the branches are large. However, depending upon where you live, you might wish to wait until after next year's blooming period to do that. If you live in a cold climate, the new growth that comes from a pruning needs to harden off before freezing weather.
You could probably safely cut it back by a third right now no matter where you live and it would not kill the plant. You would also enjoy the flowers next year. Buds form on old wood from the previous year. So it is your choice.