Package's priority is decided according to the order in /etc/pacman.conf.
Can I change the priority of single packages?
For example I have the world repo with higher priority than extra but I'd like to get a package from there instead.
Put the name of the repo in front of the package as in:
pacman -S extra/PACKAGE
You can also add the package to 'ignore' in pacman.conf. So it won't get upgraded automatically but you'll get a warning when an update is available.
Also you can add
VerbosePkgLists
to the # Misc options part of pacman.conf and you'll always see which repo's are being sourced from for each package on install or upgrade.
It would be nice if that would be done automatically but it is not a big deal.