Astronomers have found that the magnetic field strengths near supermassive black holes can be as strong as their intense gravitational fields. In fact, these magnetic fields are able to expel material from the vicinity of the black hole to form highly energetic outflows called ‘jets’.
However, this process is not acting on material that has already passed beyond the black hole’s event horizon (where the gravity is so strong that not even light can escape). Such material would need to be accelerated to at least the speed of light to escape, and Einstein’s General Relativity shows us that this would require an infinite amount of energy. No magnet, however powerful, could provide this.