Completely erasing flash

You could try uploading a full package with softdevice + bootloader + application on board, as the extra space requirement might erase user data area. Here’s one full package: https://drive.google.com/open?id=0B0x2wz-pfcjwb29KVzlIdXJjYmc. Flashing that package is somewhat risky, as accident while updating softdevice / bootloader might brick the tag.

It might be possible to make a program to erase area between softdevice and bootloader too.

If all else fails, a dev shield can be used to reprogram the tag.