Connect iPhone to Mac Without Charging
After some Googling around, I don't believe this is possible. 

Though you can disable USB ports by unloading kexts, this would disable USB port functionality altogether: it would neither supply power nor read any USB connected input device (it may still supply power, but definitely would not recognize any input). 

An alternative option would be to purchase an AC powered USB hub (for example) to reduce the power being drawn from your built in USB port when a device is connected to the hub (and hub is connected directly to AC + your machine). That way, though 500 mAh would still be allocated to the port, the iPhone would be drawing power from the AC connection. Thus, you could still connect the phone to your machine and have it recognizable via the hub without drawing power exclusively from the machine's USB port. 

Related:

Disable Specific USB Ports / Restrict Data Transfer from USB Ports