You wouldn't prune most of them, but it does depend which one you've got. If you have an acer palmatum dissectum variety, they are slow growing, don't get very tall and do tend to spread outwards, like a sort of bonsai tree or shrubby tree. Even if you prune it back, its growth habit remains the same, that is, mostly sideways with a slightly lesser increase upwards. Other Acers grow straight up, even the ordinary Acer palmatum gets tall as well as spreading, so I'm afraid it sounds like this is a case of wrong plant, wrong place.

Thank you very much I'm not very good with names of plants but will try to find the name of the one I have thanks.regards Lynda

Have a look at this photo of an acer in my garden which I've pruned to be taller rather than wider:

http://www.growsonyou.com/photo/slideshow/269287-acer-aureum/member/louisa15

It wanted to be a big wide shrub, but having a small garden it would have looked out of proportion so I removed the lower branches which were growing out rather than up, one each winter, and encouraged upward growth. As has been mentioned, they are slow growing trees, and mine has been in the ground now for over 20 years, it's not a quick job!

These acers are multi-stemed shrubs and so do grow outwards as well as upwards. We regularly tackle ours and cut out quite a lot of the lower foliage so that we create a canopy and can grow shade loving plants beneath it. You have to be quite bold and it can be risky!!!!

If you do decide to do any pruning, it should be done when the tree is dormant, as cuts can bleed badly if the tree is in growth

Thank you so much all of you who responded to my question it as helped a lot I'm new to this so I need all the help I can get regards lynd123