Added the ability to change items added to the : Settings.AllowStack[] list from a WHITELIST to a BLACKLIST, useful if you only want to prevent a few items from being overstacked. This list defaults to a WHITELIST, and can be changed by setting the setting:
Settings.StackWhiteListMode: FALSE, or by using the in game config editor /istack and clicking on the RemoveOvestackedItems category.

Further Explanation:
Say you ONLY want to prevent diamond_helmets from being overstacked while allowing everything else, the config values you would need to change would be:

This would instruct IllegalStack to treat the AllowStack list as a BLACKLIST instead of a WHITELIST, and remove ONLY overstacked Diamond Helmets... Keep in mind if you change your mind later and want to revert back to original functionality you'll have to set StackWhiteListMode: to true, AND remove anything you've added to the AllowStack list or IllegalStack will think you want to allow those items to be over stacked.