That is unbelievable.
It's unlikely.
That is unbelievable.
This isn't true.
That isn't true.
There is no denying the fact.
That [It] can't be helped.
That's unreal.
That's unrealistic.
That is improbable―not likely―(未来の事なら)―unlikely to happen.
本当のはずがない.
That's not absolutely certain.