preventing transmission-daemon from auto start?

On newer versions using systemd it is a bit different.
Check if it is enabled with:
systemctl status transmission-daemon.service
Which will return something like: transmission-daemon.service - Transmission BitTorrent Daemon Loaded: loaded (/lib/systemd/system/transmission-daemon.service; enabled;

The enabled; means it will start on boot. To stop from starting on boot run:
sudo systemctl disable transmission-daemon.service

Now check the status again: transmission-daemon.service - Transmission BitTorrent Daemon Loaded: loaded (/lib/systemd/system/transmission-daemon.service; disabled;

It should now be disabled;

Check /etc/default/transmission-daemon config file.

In it, set ENABLE_DAEMON=0, and it won't autostart.