Can you target Ruin Runner with Falling Star?

No, you cannot target Ruin Runner with Falling Star.

Ruin Runner has the ability: "I can't be chosen by enemy spells and abilities."

Because Falling Star is a spell that requires you to choose targets (as confirmed by Rule 352.14.a: "If a card specifies that an amount of damage may be split among some number of Units, then each Unit chosen is Targeted"), it is blocked by Ruin Runner's protection ability.