As with Forsythia, the advice is to prune out flowered stems, down to about a third or quarter of their length, or take them out altogether. They can get a rather gawky shape over time though, so pruning out a few stems at ground level will encourage new growth from the base; how many (if any) you take depends on the size and age of your plant.

Seems a little late to be pruning Deutzia - mine was over by mid June, and pruned before the end of June - when did yours finish flowering? If it was a month or more ago, pruning it back quite a bit now means the plant won't have much time to put out new growth - and its the new growth that produces the flowers for next year.

Thanks Bamboo, mine finished flowering about beginning of June like yours, will just chop a bit off those branches like you said for this year will know what to do next year thanks for the advice.

Punsa - you may find these two blogs in pruning useful:
http://www.growsonyou.com/AndrewR/blog/316-pruning-1-why-prune
and
http://www.growsonyou.com/AndrewR/blog/318-pruning-2-when-to-prune