R..
My grandma does this... and it's horrible because her breath is something special in the worst possible sense of the word. Lol. I started getting her to back off by perfecting the art of spit talking (only with her) where I would actually TRY to let little specs of spit fly out when I was talking. She backed off REAL quick!!! LOL! Ooor, you could try talking louder. It could be that she is having a hard time hearing you, so she moves in to make it easier. (I would probably try that first before spitting at her... lol.)