Where are you located?
J&P's response is basura.

You can prune the laterals back to within a few inches of the main canes.

In Southern California, yes, 'Sombreuil' can and will grow 12-15-ft canes,
and it is best grown on an arbor next to a fence, to provide growing room.

It will also repeat best if it is deadheaded.
You don't HAVE to deadhead it, but you will get better repeat if you do.

Jeri

You probably shouldn't prune, but you should train the canes and tie them back or onto a support. That way you will be able to easily train the laterals.

Of course I don't take my own advice and I have a 3 year old Sombrieul that is in the same shape as yours. I plan on cutting it back to 4-5 canes and taking them down to no laterals and tying them back.

I just did a similar type of pruning on two other climbers and they look great a few weeks later.

You never prune a climber until after it has bloomed in that first spring flush. You may, however, cut them back one third if they get out of control. Cutting back and pruning are not the same animal. It's OK to cut back to keep them in bounds, but pruning a climber only takes place once a year.

My DH takes hedge clippers to the top of our Sombreuils, after
their first spring flush.
Lickety-split, they come back with another flush of bloom. That's happening now.

But, of course, these are 20-y-o plants.

Jeri

Hello everyone, thank you all so much for your responses! I think I'm in zone 5. I live in Northern CA in Sacramento area, so it's considered the upper end of the Central Valley. We have very hot, dry summers, and cool, wet winters, although we have had a drought for the past two years. We do have frost in the winter, and freezes sometimes, getting down into the 20's at night.
anyway, I'm somewhat confused by some of the answers. Some posts say to prune just the laterals and do so in the winter during dormancy (which is only like a month here. Spring begins in late Feb.) But roseman said to prune only once after the spring bloom. Sombreuil is a repeat-blooming hybrid, and I thought the puning after the spring was for once-flowering climbers, the ones that bloom on old wood?

AVM, in California, you should have no problem cutting Sombreuil back after its
spring bloom.
Cut the lateral canes back as much as you have to.
It should come back strongly with another cycle of bloom.

I would, if possible, avoid shortening your main basal canes much during the growing season
though, sometimes you gotta do what you gotta do.
Roses are tough.
Sombreuil will adapt.

If you have not visited the Sacramento City Cemetery's Historic Rose Garden,
(1000 Broadway)
I urge you to do so.
Go on a Saturday, and you'll likely find a crew of volunteers working,
they'll be happy to show you the way THEY prune climbers.

They will also offer a guided tour of the garden on Sat., Sept. 20,
and later, they do pruning demos.

You can learn more there than anywhere else in your entire area.

Jeri