You don't need to bust out the slab. And, if you tried to fill the pipe with concrete, you could end up with some of it going down far enough to cause an obstruction.

There are various kinds of mechanical plugs that you can insert into the drain opening. These plugs have a nut on top that you tighten and it causes a rubber seal to expand and seal the hole. If you want the nut to be below the floor level, you just need to look for a low profile model, or cut the bolt on top as short as possible. Below is a video showing the general approach.

Here is a link that might be useful: Sealing toilet flange