The wifi connection is currently handled by netctl in /etc/netctl/wlan

Description=ethoscope_wifi network
Interface=wlan0
Connection=wireless
Security=wpa
IP=dhcp
TimeoutDHCP=60
ESSID=ETHOSCOPE_WIFI
Key=ETHOSCOPE_1234

This can be changed to

IP=static
Address=('192.168.1.15/24')
Gateway='192.168.1.1'

and to disable wifi when eth0 is connected

$ sudo vim /etc/netctl/interfaces/net0
#!/bin/sh
ExecUpPost="systemctl stop netctl-auto@wifi0.service" || true
ExecDownPre="systemctl start netctl-auto@wifi0.service"
or
 
ExecUpPost="ifconfig wifi0 down" || true
ExecDownPre="ifconfig wifi0 up"