If you want to hide a directory listing, please put a blank index.html in the folder. Not sure about the latest Tommy (I'm going to test it once my test account finishes setting back up), but the last time I messed with this, the Indexes option was supported in subfolders but not domain roots. Plesk doesn't support it properly.
The settings that can be changed through .htaccess are limited to specific settings as well. If you use an invalid one, it'll either be ignored, or produce a 500 error with a message in the log explaining the issue.
Getting the directory listings working at all was a major effort on Krydos's part. Our old cPanel servers had them visible by default, and all of us staff-side wanted that back because it reduces support cases (you'd be surprised how many people contacted us about the 403 error that you get if they're turned off, or because they didn't delete a default index.html file when we included one) and it makes spotting abuse easier (phishers notably tend to be stupid and not hide the listings, so we can literally just flip through them, see "my_phis_site" sitting there and ban them).