@hinterlands: Probably the most straightforward logic inverter would be via the Voltage applet set to 5V in the Gate-Off mode. That’ll send 5V as long as the gate is low, and then go to 0V when the gate goes high.
You could also use the Logic applet and mult your signal to both inputs of NAND.
The way to do it with the Threshold Logic Neuron is to set one dendrite to -1 and the axon to -1. This way, the axon fires when there’s no input (since 0 > -1), but will shut off when the dendrite input goes high (when -1 == -1).