Sceptimatic: You see to be very misinformed and have an odd way of looking at things. I will try to explain in a way where you can test this out for yourself.
Imagine you are on a small raft on some water, your feet are fixed to the raft, and you are carrying a medicine ball (or some other kind of reasonable weight).
There are no other forces, no waves, no wind etc. acting upon you. So there are no other influences.
You now hurl the medicine burl as hard as you can, to the left for example.
Surely you can understand as you do this you, on your raft, will move to the right. This occurs because of the conservation of momentum.
From what I can see, you seem to think that hurling the ball to the left would 'push' against the air and that is why you move. This is incorrect simply because of what would happen if you did the following:
The scenario is the same as above, but this time instead of the medicine ball (or other weight), you have a circular shape - the shape corresponds to the same shape as the medicine ball.
If you were to quickly thrust this flat shape out to the left you will not move to the right in anyway near the same manner as the above scenario. This is because the force exerted by 'pushing' against the air is negligible compared to the force exerted by conservation momentum.
Of course conservation momentum is determined by how fast you throw the ball. If you just give it a little throw you will not move as far in the opposite direction as if you were to hurl it. Momentum is defined as mass times speed, so it is determined by how heavy the thing you're throwing is, and how fast you throw it. If you try this with a tennis ball it wouldn't work because a tennis ball just isn't heavy enough.
This is the principle of how rockets work, and can work in a vacuum.