You know when you have many items stacked in the inventory, such as gems, they join in a single item slot, that has a little blue triangle on the top-left of the item's icon, and when you click the triangle, it opens another menu, of all the stuff in there, and if you drag the triangle you move all the stuff at the same time?
So yea, click that triangle, take out all the items you can (probably all but one) and put them somewhere else (treasure chest, fridge, another sim's inventory). When there is only one item left (the unremovable one), you drag all the other ones back. This time you should be able to move them.
I don't think that made it more undestandable, did it?