Is it save to delete users from the database?

Try it and tell us what happens!

/s

Honestly in my experience messing with databases in production is not a good idea. I recommend making a test instance to see what happens. The alternative is to backup the database and try it. My guess is that it will work, and there shouldn't be a huge problem from it, but there might be some strange problems that pop up from it.

I had to do that earlier, and haven't had any issues. Just make sure you remove them from both person and local_user tables. They're linked on person.id = local_user.person_id.

I exported the user's row data from both tables before deleting them just to be cautious.