Open FileMaker Advanced without opening the file and enable the script debugger. Then open the file. Use the script debugger to abort the script.
If you don't have advanced:
Create a new file. Set up a script and an external data source reference such that it performs some script other than this start up script in the original file. This will open the file without running the start up script.
BTW, these two methods for accessing the file are two reasons why "roll your own" security systems aren't really secure.
the On Open script can be stopped with cmd-. on Mac and with Esc on Windows if it does not contain the "Allow user abort [off]" script step.
Just press these keys like there is no tomorrow while opening the database.