You did not say what kind of hydrangea or where you are so I assume you mean a mophead or lacecap that blooms only on old wood. if you want to see blooms in the Spring, it would be best to prune it after it has flowered in Spring 2016 but before the end of June 2016.

For more info, go to the link below and scroll to Method One:

http://hydrangeashydrangeas.com/pruning.html

You may want to think about transplanting it elsewhere if it is a big problem for you though.

The above would apply to an oakleaf or serrata hydrangea too.

Can you add a photo of the plant including a closeup of the leaves?

Thanks NH and Luis. I'll try to remember to do so tomorrow morning, but I'm leaving for the weekend so may well forget.

Here is a pic from the internet of the type

If it looks like the photo, it is a bigleaf hydrangea, Hydrangea macrophylla. Pruning now will eliminate (or at the very least, greatly reduce) any flowering next season. And it is only a temporary measure at best - as you have already seen, it will regrow to its former size and rather rapidly.

To prevent the need for future hard pruning (and the seasonal loss of flowers), it would make the most sense to relocate this shrub to an area where it is able to grow unimpeded and without need of pruning to keep size in check. If that is not possible, there's a couple of other ways you can approach this.

1) Rejuvenation pruning: cut back about 1/3 of the stems - pick the oldest - back to the root crown (ground level). Do the same in each of the next two years. Over the 3 year period you will have removed all the tall old growth, which will have been replaced with fresh, vigorous new growth, and will have reduced the overall size of the plant. But only for a period of time. It will regrow to its former size. And you will also maintain most of the flowers.

2) Hard pruning: cutting back all stems to a low, manageable framework (10-15"). You will lose all flowers for at least a growing season but the plant will also hold to a smaller size for at least a couple of years. However, eventually the shrub will regrow to its former size.

I typically consider macs and serratas "no prune" hydrangeas - other than attempting to control size, there is no reason to prune these types except to remove any dead wood. And since pruning to control size is so tricky to accomplish while maintaining flowering potential, the best advice is to site these plants where their size doesn't matter. If a hydrangea is the desired plant for that spot, there are compact or dwarf forms that may fit the space better.

Thank you! I think relocating it is a great idea, and I have a great spot for it.

Appreciate the help.

If your bigleaf is the kind that begins to exceed 5-6' in diameter, then yes, locating it to an area that can accommodate it is the best for both you and your plant. Typically, getting ones that don't exceed 3-4' are best for medium sized gardens. Then there are dwarfs where we would consider ones that grow only 1-3'.