The inventory item manager is a very useful device, but there isn’t an option to directly disable it. However, it is possible to override an older item manager by turning on a new one that doesn’t have the old one’s settings.
First, you have to make sure that in the new manager, “Use by Default” is set to “No.” Then, have something that will trigger that newer manager, like a button. Adjust your newer one’s settings to your content and you’re all set. Note that for some reason, you can’t override a manager with an item limit using a manager without one. After you turn on the new one the old item limit would still be there even if you set “Set Maximum Amount” to “No” on the new one. Fortunately, this isn’t a huge problem as the largest number you can put as the limit is 100000000000000.