Once you pull the toilet (the two nuts on either side), you'll find it mates to what is called a "closet flange." This should be set fairly close to the floor level (usually protrudes a slight amount). To remove that completely is going to take some work. If you can get to the floor below (like a basement), you can just cut it off below the floor. Otherwise, you're going to have to cut away some subfloor, remove the flange from the top, and replace the subfloor. Either way it's pretty involved, and while reversible, not particularly easy. The "repair plug" that was mentioned above is a much better idea.