Be persistent in digging it out and in the worst case, use a herbicide if needed.
I battled torpedo grass since moving to my home in 2004. My neighbor's yard is apparently a nursery for the stuff. I removed a 40-year-old 40 some foot, Viburnum hedge between properties. I dug out the grass and volunteer oak and palm trees growing in there in its midst. The gardening bed was made using landscape fabric, mulch on top with crepe myrtles every 6 feet. Almost immediately, torpedo grass began pushing through the fabric and mulch. This was my first experience with torpedo grass. I cussed, pulled it out and pulled more for months. After 2 years I removed the fabric and once again dug out what seemed like roots the size of a pencil and some up to 8 and 9 feet long from around the crepe myrtles and now a royal palm at the end of the bed. It seemed to use the 'professional landscape border' from the home depot as a root highway to spread even faster. And it still came back and I've pulled it many of my gardening hours but I am winning the battle. It is coming back only in a couple of spots now. And I have only sprayed a herbicide maybe 6 or 7 times over the years. I hope this helps those who have the misfortune of running into this weed.
