I have used

UPDATE person
SET deleted = 't'
WHERE id =nnnn;

You can get the id’s from:

SELECT id, name, published
FROM person
WHERE instance_id= 1 and deleted = 'f'
ORDER by id;

I have not searched but shouldn’t they have some database schema documentations such as ER diagrams?

This probably needs verification, so run at your own risk!

The SQL statement to delete would be: DELETE from person where name = '...'

person is the table of users.

name = '...' is the condition for the row that is to be deleted.

You can also modify the condition to, for example, delete a user using the id instead of their name (username).

Here’s a link to the “schema” of the person table as generated by the backend code.