Never hit with a metal hammer. I use a very small hammer with hard plastic strike surfaces. You can also use a small block of hardwood. The method is to lift the platter with one hand and strike the spindle crown with a single 'medium' blow. A prior soaking with penetrating oil around the spindle will also help.
There's a pic in my gallery of my preferred tool which I will post here.
The weight of the little hammer is only 150g.