Sonna koto nai yo is correct.
Sou na koto nai yo is not correct. If you are trying to make the first one more wordy, Sonna is short for Sonoyouna. But then it does not go very well with the rest because of the difference in style. It would be Sonoyouna koto wa nai desu yo, etc.
Sonna koto yo is correct. But this is not used as positive version of Sonna koto nai yo. It would be Sou da yo. Or funnily enough, Sonna toko yo would be used as the positive version for Sonna koto nai yo.