Sometimes an item cannot be dropped, traded or sold but can be destroyed. To destroy an item in your inventory first type /keyboard and determine what key(s) are bound to the "destroy item" command. Single left click on the item so that the item seems to follow your mouse cursor, and hit your "destroy item" keys. The default destroy item command is /destroy.
( If you have not altered your Key Commands, another option to destroy an item is left-clicking the item, and then hitting SHIFT + D )