Coins are yours at the moment they hit your wallet, and it's your decision what to do with them. If you send them back, the society would say thanks, if you keep them and people know the source from those coins, then you could go to jail.
Another option is to send them back to the address it comes from.
I would rather send them back to the address, I don't want to involved myself from criminals. Although there's a lot of ways to hide like using a bitcoin mixing services, but in the long run, it might bring harm than good to you. I'm sure anyone will be tempted to hold the funds and liquidate and enjoy it for the rest of your life, but who knows, maybe one day authorities came knocking in your door and all that enjoyment and riches you have will be replace by jail time, ouch!!!.