There is a lock option for show/hide toolbars that you should check to lock it in place.
Also FileMaker 14 introduced a checkbox in file options called "hide all toolbars" that hides everything by default onOpen.
Make sure it is set to hide and not toggle and check the lock setting, which is at the bottom left. Make it the first line of your script.
There was no answer posted when I added my comment. Timestamps show it was posted two minutes before mine, after I posted.
You got the answer twice.
Regardless of what your opening script does, including locking, if a subsequent script opens the toolbars again then they will open. My best guess is that is what's happening. Check your other scripts to see which is the culprit, and keep in mind that there may be more than one.