Yes, if you want to cut it back go ahead. It will bloom later.

Lauren W: After last winter in Z5 failure of both the lilac and forsythia to bloom could well have been the result of extreme cold. Most lilacs are very cold resistant, but not all. Many forsythias will fail to bloom after significantly sub zero temperatures. Prune both shrubs in the way gardengal suggests above. Or do just the lilac that way and cut the forsythia back to manageable size.

I am well aware of how aggressive this clematis is. I made the mistake of listening to a suggestion that I use it as groundcover in a certain situation. I changed my mind about five minutes later, but have been digging out (with a spade, not a trowel) the seedlings ever since. I think this obelisk will be a one-season thing. I am wary of creating too many fussy plantings that need surgical precision to control lest they become landmines. What if I wanted to go away some September? Or the unsuspecting next owner didn't know how important it was to remove every blossom before it went to seed?

It is never too late to prune a clematis :-) Yes, it could delay or alter bloom time but in a mild climate with a long growing season, it is not going to have much of an impact. When I worked for a wholesale grower of clems, the vines used to get cut back routinely every month or so just to keep the pots/vines from becoming tangled up with each other. All this did was to encourage more basal stem growth, generally considered a good thing :-) Never affected the flowering to any significance.