First off you have to find this....
Code:
public void dropItem(int droppedItem, int slot) {
	//	misc.printlnTag("droppeditem ["+playerItems[slot]+"] which is ["+(droppedItem+1)+"]");
		if(playerItemsN[slot] != 0 && droppedItem != -1 && playerItems[slot] == droppedItem+1) {
			ItemHandler.addItem(playerItems[slot]-1, absX, absY, playerItemsN[slot], playerId, false);
			//createGroundItem(droppedItem, absX, absY, playerItemsN[slot]);
			deleteItem(droppedItem, slot, playerItemsN[slot]);
			updateRequired = true;
		}
	}
Now...
Replace it with this....
Code:
public void dropItem(int droppedItem, int slot) 
{
	boolean droppable = true;
	for(int i = 0; i < undroppable.length;i++)
		if(droppedItem == undroppable[i])
			droppable = false;

	if(playerItemsN[slot] != 0 && droppedItem != -1 && playerItems[slot] == droppedItem + 1) 
	{
		if(droppable)
		{
			ItemHandler.addItem(playerItems[slot]-1, absX, absY, playerItemsN[slot], playerId, false);
			deleteItem(droppedItem, slot, playerItemsN[slot]);
			updateRequired = true;
		} else {
			sendMessage("This item is not droppable, sorry :(");
		}
	}
}

public int[] undroppable = {1042}; // edit this to determine the non-dropables ^_^!
Change the numbers within the array to change the droppables...

Yay