Someone I don't know has randomly sent me 18,500 Bitcoins.

Is there a way for him to chargeback?

Should I call the cops?

It did not happen to me but neither may be with you, but here is what you have to do if it happens by coincidence

The first thing is Dont Panic

Take a deep Breath

Now Identify how to en-cash

Don't Post On social media that you have lot of bitcoins
Delete your Social Media accounts

Use Local Bitcoins to exchange them to cash and prepare for your new life

Throw away your phone number and cellphone. Also your computer.

Firewall your network and communications

Buy an anonymous Bitcoin debit card and use it to cash out the money in the future. It will work in most places of the world, at any ATM.

Move to another country immediately, and do not leave any traces behind you. and I mean, any traces at all. Not even fingerprints.
Use https://btctrip.com/ or Expedia to book flight tickets with bitcoin

When you move to another country, buy a small house somewhere in the mountains where no one lives. (important: register the house on someone else’s name.) Stay there for at least 2 years, and only go outside with the SPFX mask on.
After two years of your complete disappearance, move to a country of your liking and continue living your life normally, but don’t ever return to the country you lived in when the Bitcoin was sent to you. The person who sent it will probably find a way to figure out where you lived one way or another and will start looking for you. That’s why it is important to disappear for 2 years without any trace of your existence, and then continue living in another country where no one expects you to be.

Don’t talk to your friends about what happened, don’t talk to your girlfriend, don’t tell anybody at all what just happened. Simply disappear without any trace, and don’t tell anyone about where you’re going. Just go, now.