Either:
A) Right-Click bag icon = Bag is locked and nothing can be added to it.
or
B) Option Menu: Looted items go to the first available bag slot
I try to store my potions, augments, alternative gear (resists, HP), Scrap, Dynamite, etc. all in one bag to keep it organized & separate from the junk I loot… but not only does it end up filled with loot overflow, but if the bag is open it’s the first secondary bag that loot goes to!
Option A would be ideal, but Option B would work as well that way we could simply move all of those potions, etc. to the last bag and only loot to it when the other 4 bags are full.