this does not seem to work anymore. I have tried changing this and escape still exits PIE.
EDIT: After some digging the Escape key is now hard coded into the source. if you want to disable this now you have to use a source build and edit the source.
the file is located at Engine\Source\Editor\UnrealEd\Private\Kismet2\DebuggerCommands.cpp
you want to modify the following line:
UI_COMMAND(StopPlaySession, "Stop", "Stop simulation", EUserInterfaceActionType::Button, FInputChord(EKeys::Escape));
I changed mine to:
UI_COMMAND(StopPlaySession, "Stop", "Stop simulation", EUserInterfaceActionType::Button, FInputChord(EModifierKey::Control, EKeys::Q));
and now escape no longer exits, control-Q does.
Hope this helps OP and anyone else who finds this