Damian Gostomski

The Imposter plugin allows site administrators to take on the role of another user. This is useful for development and testing purposes, as well as the ability to troubleshoot user issues without needing the user password.
It will add a link for each user in the users list, which will log the administrator as that user. They’re then logged in as that user and are no longer an administrator, until they logout of the user session, at which point they’re logged back in as themselves.

Imposter

Allows administrators to take on the role of another user for development, testing and troubleshooting purposes without knowing the users password.

Blackbam

Prevent users from logging into your WordPress installation for a certain timeframe or permanently. Works also great with the multisite user management area.
The locked users are easily manageable within the users overview page. The settings are within the user edit pages.
No user can lock himself. No user with a lower role can lock a higher user and administrators in a network can not lock super administrators.
If a user is locked, he is instantly logged out of any session until the lock expires or is removed.
No useless overhead, no ads. Just a tiny, but very effective plugin to keep your website secure.
Use cases:
<ul>
<li>you do not want to delete a user, but you want to make sure he can not access the site (at least for a certain timeframe)</li>
<li>an employee leaves your company and access should be removed, but you want to keep his user as an author in the system</li>
<li>you want to punish a certain user which did bad things for a certain timeframe</li>
<li>you want only few persons to have access to your WordPress site in order to minimize risk of incidents</li>
<li>… or maybe some other use case</li>
</ul>
<h3>Feature requests</h3>
Currently none.

Simple User Locking

The motivation behind writing this plugin was very practical - how can I lock users within my WordPress site without deleting them. During the time writing this it has already been used in live environments for two months, without any complaints - doing what it should do. Therefore I am satisfied with my initial work.

Already successfully used in live environments

Prevent users (like e.g. ex-employees, rule breakers or spamers) from logging into your WordPress installation for a certain timeframe or permanently  …

Imposter vs Simple User Locking