Since those are fixed pitch helicopters, there is not much you can do except what has ben mentioned before, practice, practice. Don't let the helicopter fall, fly it down to the landing spot.

You could try to come in low while still flying forward. That will keep the turbulence behind you and not allow time for the vortex to form. The longer you stay hovering on one spot, the stronger the vortex will become.

Otherwise, when you transition to collective pitch, you will see less of this effect as you allow the power of the rotor to remain fairly constant, and only change the angle of attach of the blades. The vortex will still be there, but the control on the helicopter's descent will be much more controllable.

Rafael