Change what I circled to 0 for items that have effects during battle:
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.