うそだ isn't really a serious accusation. How would you say 'you're lying' more seriously?
It seems to me that うそだ is already strong enough, when said in normal tone (or more serious, of course), because we don't usually use in that way. Another possible strong expression would be
それはない。 (That can't be true.)
Perhaps you should compare with more casual accusing expressions such as う(っ)そだ〜 うそでしょ？