I assume you mean a spirea. I think you either prune immediately after flowering or be contended with fewer flowers if you prune at the end of summer because flowers develop on last year's growth. I would leave the nest be and put up with a big shrub.
You can't prune it with a bird's nest in it. For future pruning follow down the flowered stems to where you can see a new shoot developing and cut there. To renovate you can remove a fifth of the old stems at the base. Spirea Aguta known as bridal wreath
I have worked as a Gardener for 24 years. My latest garden is a new build garden on heavy clay.
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Luxembourg