is there a way to eliminate the effect of earth's magnetic field?

You can get rid of the effect in a local area by insulating the area against magnetic fields. I assume you mean eliminating the effect globally, though. Current theory states that the magnetic field is generated by the movement of magnetic iron and nickel at a different rate than surrounding layers. Thus, planets which have a solid core have no magnetism, because the entire solid planet rotates together. In order to get rid of the magnetic field, you would have to either cool the outer core to solidity, or somehow regulate it's rotation such that it was equivalent to the rest of the planet.

It depends over how large of area that you want to cover. If you want to eliminate it's magnetic field over the entire planet, you couldn't do it. However, if it's a small area, you would be able to arrange magnets so that the total magnetic field is very small. In fact, there are areas around any magnet that have a magnetic field close to 0 due to canceling with the earth's magnetic field.

Yes, you can setup a "nulling" magnetic field. All you need to do is make a similar magnetic field with the geometry such that it counters the ambient field of the earth.

This is really hard to do over very large areas, and you have to be on your toes when you do it as the magnetic field of the earth varies with time (influence by the solar position)

you cannot eliminate the effect of earths magnetic field but there is a way to decrease the earths magnetic field.

An iron body, whose permeability is thousands of times greater than that of air, absorbs magnetic flux lines. This is the basis of magnetic shielding used in laboratories and in electronic circuits.

Nowadays materials of very high permeability are manufactured.

By constructing large hollow cylindrical walls with those materials, the earth’s flux can be made to almost zero inside.

You could certainly reduce the effect significantly by putting some distance between the earth and whatever it is that you're trying to protect.

Alternately, you could use materials which aren't magneto-reactive.

Stop earth from rotating or create an oposite anti-magnetric field to equalise it

Stop the earths core from rotating

i do not know about eliminating or counter-acting but if we could harness it in some way we could have an indefinite source of energy what do you think

Stop the earth from moving or blow it up?