Long story short: the number of consumables/scrolls/grenades/arrows available for use during a single combat encounter should be limited in a similar fashion to how memory limits the number of abilities available.
The reason for it is quite simple: characters who rely primarily on their abilities, are unfairly disadvantaged when compared to characters who instead rely on items(especially true of characters with Ranged weapons and special arrows), because there is currently no limit(beyond the quantity owned) to the number and variety of those, that one can use.
While this obviously does not really come into play in a controlled environment such as the Arena, it remains a valid concern in general.
As per the title of this thread, I suggest this be done through an item belt: a set number of slots that the items being used must be put into before they can be used in combat. Each slot holding a limited quantity of a given item, and the able to be refilled in combat in a fashion similar to equipping a weapon(though without a talent to bypass the AP cost).
Talents could be made available to expand the number and capacity of the "Item Belt" slots.
