I can't say for sure if it's true in Florida, but here in East Texas, pine trees grow like weeds and it's a yearly chore cutting them all down in the spring. Once cut, they are dead, and what's left will rot away fairly quickly. Just keep on bush hogging everything. I have areas where I'm winning the battle, but I've also been spraying 2,4-d in those fields trying to cut down on the other weeds in there and improving the quality of the pasture grass.
Do you have sweet gums? they are worse because the roots will remain alive after cutting down the tree, or even digging it out. If there is anything left in the ground, it will sprout another tree. They are never ending and ten times worse then pines!!
Eddie