Hi Andrea,
I whack my mine back when it needs it all season because I have been training mine as as standard. It has not hurt bloom at all. In fact I get a second bloom later in summer.
Here's a photo from last spring. You can see how small it is , but still covered in bloom.
{{gwi:233751}}
So go ahead and prune out what you want gone : )
Remy

Which kind of wisteria do you have? Chinese (Wisteria sinensis) or Japanese (Wisteria floribunda)? I am more familar with the Chinese one, although I added a Japanese one last year so it will be interesting to compare differences as the Japanese one grows. Both of mine are grown as 'trees' or perhaps shrub would be more accurate in th case of the Chinese one.

Prune, prune, prune is the number one rule for me! I planted the Chinese one in spring 2000 to grow as a vine on the front porch; rapidly concluded that was not a good idea! Moved it to the front lawn later that summer to start training it as a tree. Prune, prune, prune... The first bloom happened in 2002 in July. It didn't do a proper spring bloom until 2006. It is vulnerable to late spring frosts killing flowerbuds - it had flowerbuds in 2005 that were killed in an early May frost. Ditto in 2007. This year I lost about half the flowerbuds to frost but still had a nice, but late, show. It was so cold this spring that it bloomed more like a Japaenese wisteria (i.e. flowers appearing after leaves have emerged) rather than a Chinese wisteria (i.e. flower appear before the leaves emerge), but it was pretty impressive anyway. The Chinese wisteria has reliably put out a summer flush of blooms every year since 2002. The more you prune it, it seems the more summer flowers you get. I gather the Japanese ones are less inclined to do the summer flush (perhaps because they start blooming later in the spring...?) so it will be interesting to see how they compare.

No matter which one you have, I wouldn't hesitate to prune the heck out of it. I don't do the bare wood pruning that is usually recommended because I'm too afraid of pruning off flowerbuds by accident. (I can now reliably recognize the flowerbuds on bare wood for the Chinese one but I'm not sure yet what they look like for the Japanese one - and I'm sure it'll be a few more years before I find out!) But, once new growth starts, I cut all those long whippy growths back to 4-6" unless they are growing somewhere whrere I want the plant to fill out more.

I bought the telescoping long-arm pruner from Lee Valley (essentially secateurs on a stick!) and that works great to reach the top bits! Our 'shrub/tree' is now about 10' tall and additional help was definitely needed. The pruners do the trick nicely. If you want to do a radical prune to reduce the size by cutting old wood, you will likely need a pruning saw - but once you've got it cut down to the size you want, the long-arm pruners will easily take care of the new growth.

Some pictures:

Spring bloom May 2006
{{gwi:233752}}

Spring bloom fading June 1 2006
{{gwi:199000}}

Summer flush July 2007 (The summer flush is a much smaller flush than the spring ones but still very nice IMO)
{{gwi:233753}}

Spring bloom this year - you can see how much later it was since it didn't really get started until June 1 and, by that time, the leaves had emerged.
{{gwi:161076}}

Future Japanese wisteria tree, August 2007 - all the bottom growth has now been removed (Japanese wisteria have longer flower racemes to display better when the blooms hang down from higher up.)
{{gwi:233754}}

Here is a link that might be useful: long-arm pruner

Beautiful wisteria Remy and Woody. I believe mine are Japanese because they bloom after they get their leaves. Here's a picture of my 2 giants before they bloomed this year.

{{gwi:233755}}

Now that they are done blooming they have just taken over the top of the frame and going crazy. Thanks for the advice. I will be doing some serious cutting back today and hopefully next year will still have blooms. I won't cut any of the really thick heavy branches just the smaller ones. I'll make DH climb a ladder to get the tops.

Thanks
Andrea