Drupal 7

The following module is missing from the file system

As you know, Drupal 7.50 introduced the above message for the respective errors that it detects. Drupal provides a page that deals with the problem, namely https://www.drupal.org/node/2487215.

I had to deal with this problem, and although I disabled and un-uninstall the offending module, I run the sql query (drush sql-query "DELETE from system where name = '%old_module1%' AND type = 'module';"), and, in general checked everything that I had to check, the problem persisted.

Deleting spam user accounts

If for one reason or another, your site got full of spam user accounts, the easiest way to delete them, in my opinion, is programatically. This way, when a user entity (or object) is deleted, all the other fields that are connected to this entity (like extra user fields or url aliases) are automatically taken care of.

In the different case of trying to delete all this stuff directly in the database (through phpmyadmin or mysql cli), you have to build a quite complicated query that will find all the related data and delete that as well.