I hate to say this, but the best way to avoid stealth installation of systemd is to switch to another distro that doesn't use it. It is sad, but true.
I really hate to say this, because after trying several distros for years, I ended up using Debian for more than 10 years now (the "testing" version). Since then, I never thought to switch to another distro... until the systemd plague hit the Debian community (some even like it).
There is no need to start another flame war about systemd and poettering-crapware in general; no need to mention his haughty messiah style either. The internet is full of it. The fact is, I decided not to use systemd - and Debian doesn't really give me the option not to use it.
For more than one year, I keep my Debian installation systemd-free, but as more and more packages rely on it, it is obvious I will not be able to do that forever. I am about to buy a new desktop computer and, under these circumstances, I will not pick Debian as my OS for that new computer. No time to cope with avoiding systemd in a distribution that defaults to it. Yes, I can still avoid it, but for how long? Latest addition in my blacklist was hplip, which depends on systemd. Now I have to find another way to be able to configure/use my printer (cups, apparently, but it doesn't support all my printer's features).
What about tomorrow? Another package will start to depend on systemd, so i will need to find an alternative, then another one, and so on. Sadly, migrating to another distro (quite probably Slackware) seems to me the best way to avoid stealth installation of systemd.
I understand Miroslav's motivation to keep using Debian while still boycotting systemd; I want the same too. But it seems futile to me. Since April 25, systemd is the default even for the officially stable Debian distro. We knew it was coming, now it's here. And, as systemd fanatics love to say blatantly, it came to stay. That would be ok for me, as long I could still have the option not to use it. But Debian does not really give me that option.