Short answer: The PTT button on the side is also an "escape" button.
When scanning through frequencies or memories, there seems to be no way to stop the scan mode. The manual is silent on this detail.
Here's the secret: Use the PTT button to escape from scanning modes (halt the scan).
Don't worry. The transmitter does not transmit when the PTT is enabled as the escape button for scan modes. They designed it this way, but forgot to mention it in the instruction manual. You can confirm this by placing the radio into scan mode and then press and hold the PTT button and you'll see that it is not transmitting.
The next time you press the PTT it will transmit. So be careful with double-pressing...