Spiraea arguta

Notyalca

I bought 2 recently and the flowers have gone, but there is still a lot of new growth coming through

Do I just prune as I go?  I wouldn't even know where to start pruning as some new growth is coming through right where the flowers used to be. Or do I jus leave it and let it do its own thing? 

(Picture might be hard to see, the sun is so bright!! (Not complaining or anything))

Obelixx

It's up to you.  Either leave it to grow or else prune back any stems that are too long or in the way as soon as flowering finishes.

Hortum-cretae

The aim is to provoke the plant into producing new growth from low down which will make a strong new structure for what will become a three, four, or even five foot tall shrub.  Yes, it's tricky when the new growths shoot from high on the stems like that, but you prune it to shape, removing quite a bit of the flowered growth and the plant will react accordingly.  Don't be frightened about pruning out too much. It's a spiraea, you won't kill it.

H-C

Fairygirl

I've just hacked one of mine back as I'm moving it. You really can't go too far wrong with them.

They naturally go up, and then 'fall' outwards, so prune it to the way you want, but keep that shape in mind so that it looks fairly natural. At this time of year, it'll quickly produce new growth  

fidgetbones

I prune off all the flowered stems back to a new growing shoot. If you do it now, as the flowers have finished the new growth ripens enough to flower next year. If you leave it until later, you lose next years flowers.