It's very simple actually. If you want to change course or slow down you just eject mass in another direction. To slow down you just spin the rocket around and burn in the opposite direction (or you could have two engines in both directions if you wanted, but I don't think any rocket has that.)

I'm not sure mater ejected could push back.

Ejected in this case usually means very violently pushing fuel/gas in one direction. There isn't a small gnome sitting on the engine throwing blocks of fuel into space, even if they technically could have worked. Instead we use very powerful engines.

The ejection itself provides the push back, not the matter. Once the matter has been ejected it doesn't do jack shit.

Surly the vacuum of space would just suck the rocket or thruster empty as fast as possible. . It just bugs me. Lol

I'm sure they close the valves when they aren't actively using the engine. It's not like fuel injectors in cars just constantly spray out fuel, even when parked.