"We do not know you."
Translation:Nie znamy was.
It could be "Nie ciebie znamy", but that sentence have a slightly different meaning(literally: We know not you) and just begs for an ending of "tylko kogoś innego"…
Good advice until B2/C1 level - do not separate "nie" from verb, because that usually ends up negating something else than verb instead. ;-)
That said, "Nie znamy ciebie/cię", "Ciebie nie znamy" should be accepted, since English you is both plural and singular.
It's technically correct, but it's so rarely correct that I don't know if it makes sense to accept it and make people think it's a natural sentence.
Putting "Ciebie" at the beginning gives a strong emphasis on it, so it works as in "OK, we know him, we know her... you, you we don't know. Who are you?".