Golang

One option is to use embedded structs:

type UnmarshalStruct struct {
    Foo string
}

type MarshalStruct struct {
    UnmarshalStruct
    FieldYouDontWantUnmarshaled string
}

When marshaling, you would marshal MarshalStruct. Because UnmarshalStruct is embedded, json.Marshal automatically includes all the fields from UnmarshalStruct.

When unmarshaling, you would unmarshal UnmarshalStruct and then create a new MarshalStruct like this:

var unmarshaled UnmarshalStruct
err := json.Unmarshal(&unmarshaled)
if err != nil {
    panic(err)
}

return MarshalStruct {
    UnmarshalStruct: unmarshaled
}

Although, if I may ask: why do you not want a field to be unmarshaled? You could always just set it back to the zero value after unmarshaling, which is a lot easier of a solution.