'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'...
Seriously...how could you not know that? Obviously it's 'baka'. You hear it all the time from anime or from people who watch anime... :D
This person obviously doesnt watch enough anime. Its 'baaka'....