How can I spoof my mac address on Bluestacks?
Unzip initrd.img in c:\programdata\android\ folder (by default). It's a GZIP format.

Next, you should edit initrd file with hex editor.
Find your address,
for example 
02C6400100C6400200C6400362C6400473C6400574

And Replace 02:00:00:64:73:74 bytes with your hex bytes (MAC ADDRESS)

https://www.youtube.com/watch?v=8rVumD8tmjg

Gzip initrd to initrd.img and replace original file