Sethoxydim will reduce the amount of Bahia Grass, but will not harm the lawn. Sethoxydim is the active ingredient in Vantage and Poast herbicides. The other option would be to pick the plant out by the root.
Go to your garden staore and ask they will be able to tell you what to get and how much to use so it will kill what you want it to and it wont kill your lawn or put big ugly brown patches all over it on you. They can also tell you how many treatments it will take.