Try something like this:

material flat_mat
{
ambient_red = 100; // play with these values
ambient_green = 100;
ambient_blue = 100;
diffuse_red = 200;
diffuse_green = 200;
diffuse_blue = 200;
specular_red = 200;
specular_green = 200;
specular_blue = 200;
power = 50;

effect = "
technique flatshadow
{
pass p0
{
fillmode = solid;
shademode = flat;
}
}
technique fallback
{
pass p0
{
}
}";
}

action flat_me
{
my.material = flat_mat
}

To fiddle around with materials you can download "MatsEd" at my signature. It´s a Gamestudio material editor.