How do I avoid this weekly transmission e-mail?

I don't know whats proper, but you can redirect it NULL.
> edit (nano etc.) /etc/cron.weekly/transmissionb
and add ">/dev/null 2>&1" without the quotes, after every command that you don't need emails.

Then restart cron with
>sudo /etc/init.d/cron restart
or restart whole box.

It should redirect the mails transmission sends when something happens to NULL , google for more info about cron mails. Not sure if it works with wheezy as I'm getting now ramlog mails even eith that redirecting, it did work with debian squeeze tho.
You just won't get the mails anymore.

I get a daily mdadm email that is equally as annoying, especially since I am not using mdadm.

I edit /etc/cron.daily/mdadm and it looks like this:
MDADM=/sbin/mdadm
[ -x $MDADM ] exit 0 # package may be removed but not purged

exec $MDADM --monitor --scan --oneshot

so I just do this?

exec $MDADM --monitor --scan --oneshot >/dev/null 2>&1