You've run into the biggest problem with St. Augustine. The best way to
take care of it is to apply pre-emergence products in Jan-Feb. If we have a
warm winter and the crabgrass doesn't die in the winter (crabgrass is an
annual) you can use round-up and as long as your St. Augustine is dormant
and not green, it won't harm it. You can also hit it with round-up now. It
will kill the surrounding St. Augustine. 1 week later put a plug of St.
Augustine in the middle of the sprayed area.

A product called Image will kill crabgrass in the summer. I've never had
St. Augustine so I don't know how harmful the stuff is to St. Augustine, but
the label says it's OK. It's quite expensive. A small bottle will run you
almost $30 and will only make a few gallons. I would use it in a test area
to see how it affects your grass.

If chemicals don't work, keep the crabgrass mowed short at all times.
Crabgrass grows faster than grass. When you mow it, the crabgrass has to
use energy stored in it's roots to recover. So if you are constantly
keeping it mowed down, it will have a harder time than the St. Augustine
will. Also it has to get taller to put out seed which will cause problems
next year.

Thanks for the product tip Roy, maybe Image will do the job.

http://www.pestproducts.com/herbicides/image.htm#Summer