Restart Grand Theft Auto V Online?
Solution 1:
When in-game press start(esc) > online > swap character, 2nd slot should be free and allow you to make a 2nd character.
You should have the choice between starting fresh or using the level and bank account from your previous character. You just have to select you do not want this.
I am not sure if you want to just start over with level and money, or also achievements. Achievements, such as the criminal mastermind, are tied to your account.