Well.. 'Mendokusai' is irritating or bothersome. I doubt you would want the polite form of you, 'anata', so.. You could try the slightly-rude depending on the situation and your tone 'Omae wa' or the...
'Never say' would translate to: 決して言わないでください /kesshi* te i WA nai de ku da sai/ in polite Japanese. [*: the 'shi' syllable is pronounced geminated. When read together the 'i'...