thanks for your reply.
Hmm i think cutting the USB and the 12V wire is a bit drastic. And i guess the optical result would be quite ugyl atleast for the usb cable.
So i guess there isnt another more simple way?
Could someone also maybe give me an answer to this?:
"Another question: if i cut the power to the LEDs at Vin and the arduino is still connected to 12V and USB, would be still power drawn from the 12V power maybe by arduinos voltage regulator or something like that?"