Pre-emergent is a seed killer. Once the seed germinates, it's too late.

"William W. Plummer" wrote

You didn't say where you live. If you are far enough north, you can
use "Preen" which is the same thing as a pre-emergent crabgrass killer.
It just prevents new sprouts. But if they are already there you need
a thick (3" or more) mulch. Another thing that works well is corn
gluten, but it's a bit pricey. Chemicals such as RoundUp and CleanUp
are too hard to control and you'll wind up killing your shrubs.