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Change what I circled to 0 for items that have effects during battle:
Spoiler:
By doing this, the player will not be able to use items during battle. For some items (i.e. Potion), setting Battle Usage to 0 does the trick, but for others, such as the berries, I also had to set Code to 0 in order for them to stop working during battle. I should mention that this does not take out their held item effects, so it will be exactly like the Battle Frontier.
If you care for a less global solution, FR's multi-player check is at x143d6- you'll need to write some ASM but it shouldn't be that hard to branch away from there and check a flag instead. This would allow you to stop the player from opening the bag at all during only specific battles.