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.

HasinHayder

captainhaddock

A simple and nice plugin to block existing users from logging into the admin panel by assigning them to the ‘Blocked’ user role, as simple as that. If the installation is okay, go to any user’s profile and you can find a new role called Blocked in the user role dropdown. Just assign any user to that role, and the user will be taken to the ‘blocked’ URL whenever they want to log in.
Unblocking a user is easy. Assign them to their previous role and you’re done!

User Role Blocker

A simple and nice plugin to block existing users from logging into the admin panel by assigning them to the 'Blocked' user role, as simple a …

Imposter vs User Role Blocker