If you have FMP Advanced - and I hope you do - you can turn on the Script Debugger before opening the file, and stop the script as it starts to run. Otherwise, if your script does not have User Abort disabled, you could try pressing Cancel or Escape repeatedly as soon as opening. I expect you'll need to be quick and the script not too short to make that work.
Someone on FMForums suggested I open the file with the script debugger on, which worked beautifully. Problem solved.