You choose whether or not to target a creature spell before you decide whether or not to spend {U} on Repel Intruders. You may target a creature spell and then not spend {U} to cast Repel Intruders; if you do, Repel Intruders simply won’t affect that creature spell. Similarly, if you don’t target a creature spell but then you spend {U} to cast Repel Intruders, that part of Repel Intruders has no effect.

If you choose to target a creature spell, Repel Intruders has a target. If that target becomes illegal by the time Repel Intruders resolves, the entire spell won’t resolve. You won’t get any tokens.

This spell checks on resolution to see if any mana of the appropriate colors were spent to pay its cost. It doesn’t matter how much mana of that color was spent; the effect isn’t multiplied.

If this spell is copied, the copy will not have had any colors of mana paid for it, no matter what colors were spent on the original spell.

This spell cares about what mana was spent to pay its total cost, not just what mana was spent to pay the hybrid mana symbol in its cost.