The easiest way to prevent Flash theft is to protect your file from import. Luckily, all it takes is a toggled checkbox.

Flash provides the ability to import compiled Flash files. Strangely, a Flash .swf file can be imported into an .fla with symbols and parts intact. If you protect from import, you can protect your file from being imported into a Flash file by another user.

Step 1: Open your FLA file.

Step 2: Navigate to the Publish Settings in the File > Publish Settings...

Step 3: Click the Flash tab.

Step 4: Check the Protect from Import checkbox.

Step 5: Set a password. Note that this password will be kept in the Publish Settings in case you forget.

Step 6: Press Okay to Save.

Your file is now protected from being imported into Flash. In order to further protect your file, take a look at the Preventing Decompiling section for more information.