What does it say in the Bible about people who cuss or say negative things to others?
I assume you’re referring to believers who do this. Here’s what Paul had to say about it.
“Do not let any unwholesome talk come out of your mouths, but only what is helpful for building others up according to their needs, that it may benefit those who listen. And do not grieve the Holy Spirit of God, with whom you were sealed for the day of redemption. Get rid of all bitterness, rage and anger, brawling and slander, along with every form of malice. Be kind and compassionate to one another, forgiving each other, just as in Christ God forgave you. (Ephesians 4:29-32)
By implication, believers who make a habit of acting contrary to this are at the very least showing disrespect to God and a lack of gratitude for what He’s given us. This causes the Holy Spirit to grieve, and may deprive them of blessings they could other wise have received, both here and in the Kingdom.