I think the power flag is preferred and is what I usually use (and what the documentation recommends IIRC) to stop ERC errors if pins are not set to power output (see below)
In the above you just need to put a power flag on pin 2 of the fuse and the warning should go away.
Also, you can set a component pin to a power (i.e. GND/VCC) output and no power flag will be needed.
Notice the 6V net does not have the same warning, I think as the opamp output will be set to an output.
Edit - just confirmed this works fine, so if you have e.g. a battery symbol then set the pins to power output and there is no need for flags. With a typical linear voltage regulator you would set the OUT pin to power output. This is common in all PCB software I have used, you need to make sure you set the pins to the correct type (not just power) when creating a component so the ERC can work properly. Here are the types available in Kicad:
The error only occurs (correctly) if a net has nothing to drive it, so if you have something like a power connector which has passive pins (if set correctly) it won't know they are intended for power until you tell it explicitly.
You can actually decide what you want to be told about, bu setting the table below accordingly. For instance if you wanted the ERC to throw an error if an input was connected to an input you would change the topmost box from green (no message) to yellow (warning) or red (error)