How do i separate the s75578kg fridge freezer

didnt specify fridge but no the air is moved through ducting to fridge side from freezer

as you have not put a model number, I shall assume it is a side by side fridge freezer..
if you take out the crisper, you should see a special vent that leads directly into the freezer compartment.. that vent aperture is controlled by a separate slide on the front of the crisper, or on the side just above the vent in the body of the fridge freezer.. simply close that aperture down by moving the slider in the right direction.. but if your food is already frozen, it will take a long while to thaw, even with the settings corrected.

Is the fridge part a separate system? More likely a fan stopped blowing cold air through from the freezer section. If Separate, Thermostat knob prises off end and you will see where wires and sensing phial are located.

With most units you can't do that. The fridge gets it's cold air from the freezer side. The air is blown through a vent at the top. Samsung is one kind that has a separate unit for fridge and freezer and could possible be cut off separately but most can not.

That is called a " cap tube separation " . It can be caused by overlooking a defrost problem for too long . If the coils inside of the freezer frost up , that frost , will eventually work its way out thru the back toward the compressor . When it does this , the " low side ( suction ) " , receives liquid refrigerant , instead of gas . This liquid refrigerant , will destroy the compressor internal valves , causing it to loose the low side . ( Never charge a refrigerator low side with liquid , only gas . The high side can be charged with liquid . ) A cap tube separation , is when the pressurized ( causes heat ) cap tube , becomes separated from the low side ( suction ) tube , causing the suction tube to frost up , due to the cap tube no longer attached to it . When this happens for too long , the only repair , is to replace the compressor AND repair the separation , with wire ties , thermal putty sealant and aluminum tape .
I apologize for the technical explanation , but there is no other way to explain it .

freezer is separate from the fridge and runs separately.there could be a problem with the evap motor or temp sensor

The compressor and Freon are all one sealed unit,never heard of a separated system. by turning it on you will just drain the rest of the freon.

no you can not. there are no separate defrost control for freezer and fridge. the evaporator is located in the freezer section and it is supposed to defrost automatically. what is the problem in the first place?

Sounds like an air flow problem from the freezer section to the refer section. Check that nothing is blocking the air vents, and that the dampers are operating properly (they control the flow of air). Of course if it's under warranty... call the service techs out.