I have sprirea that is way out of control
I cut them so that I can trim them to control their growth. How far back can I cut them?
Spring blooming spirea can be cut back as soon as flowering is done. You can cut these back as far as you want, even all the way to the ground; this will allow lots of nice new growth.
Summer bloomers can be cut in winter or early spring; again, you can cut back as far as you want, though people don't usually cut them back as hard as spring bloomers. Also, if you trim back the blooms when they've finished flowering, often another burst of flowers will follow. This article has more info on spirea: https://www.gardeningknowhow.com/ornamental/shrubs/spirea/growing-spirea-shrubs.htm