With this plugin you can block any bad users without deleting the account and then any time you want you can unlock the user.

1. Upload lock-bad-user plugin to the /wp-content/plugins/ directory
2. Activate the plugin through the 'Plugins' menu in WordPress or by running wp plugin activate lock-bad-user command in terminal.
3. Go to Users > All Users and you will see a new column Lock User in the table.
4. You can lock any user by choosing Locked from the dropdown and then clicking on Update user button at the end of the user settings page.