How can I customize the DefaultInput.ini?

I need to make some bindings false. The ones are: .Bindings=(Name="W", Command="GBA_MoveForward") and the same for S, A, D, LEFT, RIGHT, and LeftControl.

If I recall correctly, there's a hierarchy of configuration files in UDK.

At the lowest level you have BaseInput.ini, then DefaultInput.ini, and above all UDKInput.ini. Each configuration file adds/removes/overrides values of the one below with the following syntax:

+Something // adds a value to an array
-Something // removes a value from an array
.Something // overrides a value in an array


So in your case, if you really just want to disable a binding, you could do the following in UDKInput.ini:

-Bindings=(Name="W", Command="GBA_MoveForward")