'말다' is an negation expression usually used in interrogative or exclamation sentences. 하지 마라. --> Don't do that. -말이야(말야:contraction) is kind of additional expression to exaggerate. 하지 말란 말이야. --> I am telling you please don't do that. 사랑 한단 말이야. --> I am really love you. you know?

Don't do that ~ ex) Dont think of your ex-girlfriend (헤어진 니 여자친구 생각하지 말란 말이야) 말란말야 is an informal usage of 말란 말이야. ex2) Don't be flippant! (경솔하게 굴지 말란 말이야!) I'm sorry my English is not good but I hope it would be helpful to you :D