This is the kind of plant that even if you give it a bad haircut, it will grow back in a short time. There are several ways to prune spireas. Here is a link that mentions it. It depends on what kind of result you are looking.
The best time for a one spring bloom a year type like snowmound is right after flowering. S. japonica can be cut back anytime without loosing flowers.

Here is a link that might be useful: Spiraea nipponica

Last year I was asking for spirea advice. I trimmed them back hard last year. This year they grew back nice but they didn't flower much. Next year should be real good because they are full of new groth.