If the Developer's Delight or Teleport Probe is added to the client-side inventory without actually being in the server-side inventory and it is used, it will still work. There should be a check in the packets which checks if the Developer's Delight or Teleport Probe is in the player's hand before doing what the packets request.

I can't fix this for the developers delight item as it is fully client side. So there is no packet to the server.

With the teleport probe I can also not easily prevent opening the GUI but I can prevent the teleport. So I'll do that then

The probe abuse is fixed next release