Hi ! I would let them bloom and enjoy the show! Then cut down to about a third of its size . Then run out and look for a bigger trellis because they will keep getting bigger and fuller ! :) consider a netting or chicken wire that can be disguised with your Clem's. .
Also post pictures ! Gotta see them !

I let them drape over the trellis when it gets to the top. I also have ten foot poles with garden fencing across the back of the garage and across the back of the vegetable garden. None has gotten to the top yet but Duchess of Albany has been close.

Thanks to you both. I can't really change my trellis. It's a 32' long fence/trellis structure that is cemented into the ground. The clematis have been in there for 3 years now. I have 2 Jackmanni's and 2 Huldines. Last year I just let them drape over the trellis too. I just wasn't sure if you could cut them back or not. I think I'll just let them alone to flower, they look all tangled up on the top but maybe once they bloom they won't look so messy. I'll show pics when they are in bloom. Thanks again for your help.

If it's a fence type trellis, can you pull the stems sideways? I do that sometimes when I have horizontal space to cover and the clem wants only to grow vertically. If you fan it out sideways at the bottom, and hold it there, it will take off vertically from there, covering more of the fence, and that will use up some of the length.

opheliathornvt ~ Thanks. Yes, I've been doing that. It seems to be working pretty good. Thanks for the tip.

Yesterday I gently untangled some of the clematis stems that were growing above the neighbor's low fence and bent them carefully and wound them in and out horizontally along the fence. Some clematis are more brittle than others and occasionally there is a broken stem though not low. I do this every year.

Huldine grows to the top of my twenty foot plum tree...she is much too big for a 6' trellis....