Regular users cannot successfully run
sudo passwd root or whatever other commands they choose with
sudo. Only administrators can do this. Membership in the
sudo group confers the ability to perform any action at all as root with
sudo as well as with Polkit, which is a separate mechanism that doesn't involve
sudo. Don't give this power to people who you don't trust to have it.
This is to say that not having been given the power to run commands as root is what stops people from performing administrative tasks without authorization, including from setting a root password. People you have given the power to administer the system can set a root password. If you trust those people, tell them not to set a root password (and don't set one yourself) if you don't want one set. If you don't trust them, then the problem is not that they might set a root password, but that they already have the power to do whatever they want to your system!
To be clear, trust is not binary. You can trust people with some powers and not others, and this is even reflected in the security model, where users can use the system but do not have all possible capabilities. But people who can run any command as root with
sudo do have all possible capabilities when they choose to. This is why, when novice users ask about setting a password for the root account so they can log in as root, we urge them to consider not doing so. Doing so does not confer any power to the user that the user did not already have. It's not that we want to keep them from obtaining power. They already have that power.
There are risks associated with setting a password for the root user and thus enabling root logins, and benefits of avoiding it. I recommend you avoid this unless you have a clear reason to do so. (Note, however, that there are circumstances where it is reasonable to do so, as well as well-designed operating systems--like OpenBSD and Fedora--where root logins are enabled by default.) But enabling it is not destructive in itself. In contrast, you and any other users you allow to administer the system can already undertake extremely destructive actions by running commmands with
sudo, whether or not the root account has a password set.
You are a member of the
sudo group. You can delete all the files on the system. You can write raw data to the hard disk, overwriting what is there irrecoverably. You can access other users' files, even if they have set restrictive permissions. You can install new firmware to your physical devices. You can dump users' passwords from the shadow database and attempt to crack them, or just reset them. You can install malware that infringes users' privacy or destroys data, including keyloggers and ransomware. You can do some really weird stuff with your network interfaces. You can make the system misinform users about the security of their communications. You can cover your tracks. You can hand all these powers, and others I haven't listed, over to people who will use them for evil. When it comes to misusing
sudo or Polkit, those are the kinds of things you should be concerned about.
It is true that you shouldn't let untrusted people into your house unsupervised, but the reason why is not that they could then unlock the door from the inside and get in.*