Remove the toilet, cut the roots away, and also dig the ones out of the floor between the concrete and the pipe if you can get to them. Then make a solution of water and a pound of salt, or copper sulfate crystals, and pour it down into the gap around the pipe. It will deter them for a long time, but eventually they will return. I have had customers with two toilets back to back and it would happen to one of them, and then 6 months later to the other one, and 6 months after that back to the first one. You will also find a tendril going into the pipe opening. Pull that out along with any root mass which has grown inside the pipe.