First, I think it's important we at least understand a little bit about what "magnetism" is, and how ferrofluids interact. A ferrofluid is a suspension of really tiny magnetic particles. You can think of them as tiny bar magnets floating around. They're so tiny that under normal conditions (if there's no magnetic field), heat causes them to point in random directions. However, when you apply a magnetic field, it causes all the bar magnets to line up along the magnetic field lines of the applied magnetic field. As far as the "level of magnetism" in the ferrofluid, these magnetic particles will respond to any applied magnetic field, and when the field is removed, the particle orientation will be randomized again.

It's actually really difficult to shield things from magnetic fields, but one type of metal you could use to disrupt the magnetic field is mu-metal, which is specially designed for this purpose. You could also interfere by creating an even stronger magnetic field, which interferes with the original field.