If you dig down 4 feet and the replace all that dirt with perfectly sterile fill, you can get rid of St. Augustine. For a season or two.
There is only one product I know of that can get rid of that stuff without killing the fescue and I won't recommend it because it's safety has been called into question.
For reference it's called turflon ester.
Do your own homework, the EPA says it's safe but others don't agree and I don't feel like being a litigant.
