Noah never warned the other people. The ark was only destined for him and his family:
By faith Noah, being warned of God of things not seen as yet, moved with fear, prepared an ark to the saving of his house; by the which he condemned the world, and became heir of the righteousness which is by faith.
And, behold, I, even I, do bring a flood of waters upon the earth, to destroy all flesh, in which is the breath of life, from under heaven; and everything that is in the earth shall die.
But with you will I establish my covenant; and you shall come into the ark, you, and your sons, and your wife, and your sons' wives with you.
And the LORD said unto Noah, Come you and all your house into the ark; for you have I seen righteous before me in this generation.