Without replacing the floor with something that is much more serious than the typical wood framing and subfloor you are not going to be able to harden that floor without serious investment in time and energy.
You could sandwich a 10-14g sheet steel between the current subfloor and cover it with High Density plywood but man that is going to cost you a pretty penny and take tons of work to get the job done and all it would mean is that they would now need an axe to get to you.
Ideally you (and I am saying this without seeing the house and with no real information on how its built) would replace the floor with a concrete floor. I know that sounds like a lot but on the classic American house (stem wall then wood framing) it would be pretty easy to pour a slab in place under the existing floor. You would need to hang your rebar off the current floor, build a deck form and shore it up then cut holes in the current floor so you could pour in plasticised concrete. That shit really flows and because you are still using your existing floor you won't need to finish the concrete.
I will do the job site unseen for $75,000