This utility can erase the NOR on the EZ Flash 3-in-1. I cannot confirm that it works with other NOR-based Slot-2 cards. If you erase as little as 4 blocks, your DS will no longer show a GBA Game in Slot-2.
Once again I fell down the martial arts rabbit hole and am plumbing the murky depths of the internet's various martial arts and fighting crafts echo chambers.
If you're not actively involved and only get up to speed every few years or so, you can see the tide of public opinion flapping worse than my dong while I'm skipping naked down a hallway.