The only way I know about is deleting the folder .thumbnail and then creating an empty file named .thumbnail (this will prevent the folder being created again). However, be adivised that Album app may get laggy since it'll regenerate the thumbnails every time you open the app or use the photo widget. If you have a lot of them, your phone also may get hot and the battery will go quickly if you keep opening album app various times...
Furthermore, it's hard to know the real size of that file. Yes, it holds some space in internal memory, but it's definitely not the size shown. In my phone, for example, this file show as 870 MB, but once I delete it I get only 50-60MB of space back...
Sony Xperia M dual [C2004]
ROM: 15.5.A.1.5 (stock)
Android version: 4.3 (rooted)
Bootloader: locked (unlockable)