how to stop washer machine back flow when it it is on spin

Backflow likely has nothing to do with a blocked drain, if that is truly what you're discussing. Backflow occurs when the pressure in the drain pipe is higher than that in the washer tub, causing waste water to flow backwards into the tub. The most likely causes are that the washer is not high enough relative to the drain pipe, and that there is no air-break (AKA air gap) in the drain pipe, required by many sanitation codes.

To fix the issue at little or no cost, do the following:

• Raise the height of the washing machine, e.g. by putting underneath it a sturdy 10 cm x 10 cm (4" x 4") frame, with depressions for the legs so they don't "walk" out during sPin cycle.
• Raise the height of the standpipe, the vertical pipe connected to the plumbing system.
• Add an air gap between standpipe and drain hose, if there is not one present.