Pin 13 fires relay when powering on... How to disable?
There are three things you can do:


Remove the bootloader entirely and program the board with a hardware programmer (USB-ASP, Another Arduino, etc).
Edit the bootloader source, recompile, and reinstall it to the board using a hardware programmer (as in option 1).
Just don't use pin 13 for the relay.


Of the three options the third one is by far the simplest.