udev rules is not the answer! You need to disable bluetoothon the UART.
To do this, you need to:
Shut down your RPi Get the SD Card out and hook it to your PC. You will see a config.txt in the root folder of it. Open it with a text editor Somewhere around the top, maybe below the “kernel” setting, add: RPi3: “dtoverlay=pi3-disable-bt”
This is what I have in my /boot/config.txt:
# Additional overlays and parameters are documented /boot/overlays/README
# Enable audio (loads snd_bcm2835)
dtparam=audio=on
# mini UART
# enable_uart=1
# core_freq=250
# switch bluetooth function to ttyS0 and restore UART0/ttyAMA0 over GPIO 14&15
dtoverlay=pi3-miniuart-bt
# or disable internal bluetooth
# dtoverlay=pi3-disable-bt
start_x=0
root@HS3Pi3v5:/boot#