Orbital mechanics.

The efficient (and probably only viable) way to do this is a solar shade at lagrange point 1 (directly between the sun and earth). There's an unstable equillibrium point there were things can sit permanently with some (but minimal) station keeping. But you don't get to pick which part you're shading, you're always directly between the earth and the sun (or really the center of the earth-moon systems gravity, and the sun).

I see, thanks. I guess I was thinking of something that 'moved' with the earth, but that probably isn't realistic.

Could something be placed at such a point that then only blocked sun on the far north/south of the planet, near the poles?

Not directly. You could place something in an orbit where it only ever blocked sun at the poles I think, but it would spend the vast majority of its time not blocking sun at all.