You forgot that you need to turn on a switch after that, otherwise the player can keep getting items from it.
After the Change Items command, add a Change Switch. I'm going to assume you know how to work with switches, because if you don't, you really need to look up a tutorial.
New Event Page, under Conditions on the lefthand side, click the Switch Box. Select the switch you turned on. Don't touch anything else, and the item should disappear after you pick it up once.
You may also want to add a Message command to say that the player picked up a necklace.