Stopping halts the simulation and resets all step state and variable values. (Use Reset instead if you want to restart from the initial state while keeping the simulation running.)
Using the toolbar (button):
Click Stop Simulation in the Sequence Explorer view title bar.
Using the keyboard:
Press Shift+F5.
Using the Command Palette:
Press Ctrl+Shift+P (or Cmd+Shift+P on Mac) and run AutomationView: Stop Simulation.