Hmm no it doesn't work for Bcc fields - I'll have to look into another method.
In case anyone wants to try this for blocking emails to a specific address in To: or Cc: fields:
I tried sending a bcc email to myself and received it so I assumed it didn't work. However I haven't had any spam mail since so this must be working with BCC fields too!
1) Add a line to your main.cf
header_checks = regexp:/etc/postfix/header_checks
2) Create a new file, /etc/postfix/header_checks
3) Restart postfix
This will silently accept the email but not deliver it - if you wish you can use REJECT instead of DISCARD and it will send an undelivered message back to the sender.