How was the WannaCry virus stopped?

The Spread:

Spread to host computer through exploits in network infrastructure (since patched).

Hold Drive Hostage:

Encrypt the user's entire drive, display a message to pay up for the encryption key.

Repeat.

So a cyber security analyst who was digging through code the worm uses to spread realized something. There was a website url that is referenced in a few places. He tried to go to the website, but found it didn't exist. So he bought the domain for $10 from a site like godaddy.com and forwarded it to a sinkhole server where it couldn't do damage.
Once he set this up, almost immediately he was getting thousands of connections a second.

What happened?

The code he edited basically (over simplified) said:

1. Try and connect to the website: qwhnamownflslwff.co
2. If the website doesn't exist, keep on spreading.
3. If the website exists, halt spreading of the malware.

It was essentially a kill-switch programmed in he accidentally stumbled upon.
Note: When we say the virus was "stopped", we are only talking about "The Spread"