In most cases I would say not to prune a shrub in August but Spiraeas are such tough shrubs that you can get away with a summer pruning and your spiraea will forgive you. People come  before plants so don't feel guilty about cutting back any branches in the way of walkways. You can do a heavy pruning of your spiraea in the spring and again in mid summer to keep it under control but this shrub is  destined to grow large so perhaps the more practical thing to do is to move it to a spot where it can spread out and won't require pruning to keep it in bounds. Keep growing, Marianne Binetti