When i connect the phone to the car's head unit, the phone book is automatically being downloaded and i wish to prevent that from happening (it is not possible from the head unit itself).

is it possible to block phone book download only?

I suppose you could disable support the profiles in bluez. pbap and/or irmc (think thats right off top of head) are the ones you need to do. Search forum for pbap thread and the file that needs editing should be listed there. Once done, reboot to make it take effect.

It might be your head unit supports one but not the other so try disabling one, then the other and finally both.

Edit: out of interest, what car/head unit?

Mazda 6. i want everything to work normally though - only to disallow the phone book download to the head unit.

i don't know how to only disable when connecting to head unit, or even if its possible. all i know is how to turn disable pbap/irmc profiles.

under old instructions, step 6, just do reverse of what it says.
http://wiki.maemo.org/Bluetooth_PBAP_pre_0.46