you do not cut them back as you might a mum ...

or i am totally misreading your question

let them bloom .. then cut off the head ... as the last of the flowers are fading... BEFORE seed matures... and you MIGHT get a second lesser flush.. depending on how early fall comes..

of course.. now that i reread.. you dont mention.. what kind of phlox ...

ken

You can cut them back to prolong bloom if you like. I tried it one year but prefer not to as they bloom much fuller without it. I do deadhead and get a second bloom though.

They can also be cut back for a tiered effect. Deer come through and cut mine. I prefer to wait until after first bloom because find the blooms are smaller when pruned. For me bloom is about two weeks later when pruned.