Make your forces balance out
We need some physics here (and better graphics skills than I possess, unfortunately). Your basic force diagram for an item pivoting around a fixed point is like this:
The head of the hammer has an inwards force from its arm/handle around its pivot point, and momentum in its direction of swing. Provided the inwards force is sufficient to match its momentum, it rotates and strikes its target.
If the inwards force is insufficient because there is a rocket adding more force, the head flies off the handle at a tangent. The requested weapon is a hammer, not a catapult launched gyrojet, so no need to explore this further.
So the trick here is to make the inwards force sufficient. You could add another rocket at the head of the hammer producing an inwards force - but the net effect is a force vector between the two rockets. So why not just point your main rocket an angle to provide that vector, so it is both accelerating the hammer and providing the necessary inwards force to stop it flying away.
Now you have a hammer that can have a long, accelerating swing that should not go flying out of your hands or dislocating your shoulders. (The exact angle of the rocket relative to the striking head is left as an exercise for the engineering team. The acceleration and maximum controllable resulting speed are a matter for your medical staff.)
However, this will be a nightmare to control. If you twist the head at all during the swing, it will suddenly veer off to one side. Therefore, I strongly recommend that you use some kind of eye-tracking hardware that lets the hammer identify the spot you are trying to hit and add the ability for the hammer to self-correct during the swing to hit that spot. (Vectored thrust on the rocket or smaller steering rockets are options, gyroscopes are probably not a good idea on a rapidly rotating weapon). If this makes the weapon too complicated then make sure you practice to always have a perfect swing.
Assuming this all goes perfectly, you have just hit your target with the head of a hammer travelling really, really quickly. Now you get to feel that jarring impact through your hands and arms in one very brief and incredibly painful instant - unless your engineers are looking after you. The simplest way to join the head and handle of a hammer is to put the handle through a hole in the middle of the head. This also allows you to keep exerting force on the target through the strike. However, neither of these considerations apply here - the concept of "simple" went out the window a long time ago (relatively speaking) and the rocket will keep exerting force on the target. All you need to do is survive the jarring impact with minimal pain and injury. What you want is for the handle to end in a sleeve that fits around the head, with shock absorbers between the end of the head and the sleeve. While the period of deceleration will be very brief as the sleeve moves 5-10 cm forward, it will be much better than the jarring impact you will experience otherwise.
Final thoughts - do not try this at home. Pick the home of someone you do not like. Then let them try it while you watch from a great distance.