Unraked players in Gold placement matches.

Simple.

Why is it possible to have my gold 2 placement matches ruined by an unranked player (1 wins 6 losses in their placements)??

She fed the lane, then afk'd on the pad continuing to trash talk the team. This player has nothing to loose; and is only here to abuse others, damage their enjoyment of the game, and spoil their hard effort. They are toxic human waste, ruining the community.

Why is there not a system in place to prevent such an occurrence?

If I cant queue with a Bronze player, how can an unranked player with 6 losses be at my ELO ?

This is very clearly a flaw in the system. I fail to believe it is a legitimate and intended product of the ranked system. Check your matchmaking Riot - you have critical flaws.