When you say "force", you need to understand that force is useless in propelling Saitama back to Earth if something else isn't there to exert in on him.

On Earth, Saitama and All Might can "push back" because they are hitting air and the air is what's pushing them back.

Here's an experiment, take 2 balloons and fill them with air. Now, keep those balloons 1 meter apart on the ground(or air) and push one into the other.

What you'll find is that even though only one moves, both of them go away in the opposite direction. Just like All Might bounced off of the air after hitting it.

What you said is impossible because there is nothing else to bounce off of in space.