That is a difficult situation. There is not a good way to kill carpetgrass without killing St. Augustine. I might try using a wiper applicator to put a product such as glyphosate (Roundup is one brand) or a grass killer such as imazaquin or fluazifop by wiping it on the non-desired species here and there but not on the desired species. After a few such activities over time you'll be getting more and more St. Augustine and less and less carpetgrass. The following web site shows how to build a wiper to use:
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