[Other] Disable bag in trainer battles

Is there any way to prevent the use of items in trainer battles like in battle frontier IN FIRERED?

Which game are you hacking? If it's Emerald I'm sure you could use some clever manipulating of the Battle Frontier specials.

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.

Thank you very much I appreciate that.

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.