How to tell a girl that you want only to stay as friends?

Option 1 : Whip out your penis and start fapping. She'll get scared and leave you alone.

Option 2 : Tell her you're gay. This may not work all of the time, but it may just make her lose interest in you.

Option 3 : Tell her to f'uck off, literally. This will assure her dislike of you.

I'm sure there's more, but those usually work best.