How do i make sure a service does not start automatically or manually on systemd?

[amurleopard]

You can mask the service.

mask NAME...
Mask one or more unit files, as specified on the command line. This will link these units
to /dev/null, making it impossible to start them. This is a stronger version of disable,
since it prohibits all kinds of activation of the unit, including enablement and manual
activation. Use this option with care. This honors the --runtime option to only mask
temporarily until the next reboot of the system. The --now option can be used to ensure
that the units are also stopped.

$ sudo systemctl mask snapd.service 
Created symlink from /etc/systemd/system/snapd.service to /dev/null.

Now the even manually trying an action with this service will not be allowed, until it is unmasked.

$ sudo systemctl restart snapd.service 
Failed to restart snapd.service: Unit snapd.service is masked.