Feature summary (what you would like to be able to do and where):
The option "prevent this user from logging in" (disablelogin) should be added to both the MW block form and the Global Blocking extension. This is unrelated from the configuration variable $wgBlockDisablesLogin that allows a wiki to configure that all blocked accounts cannot log in, but having this option when this configuration variable is off (or perhaps a different configuration variable sets this flag).
If this setting and autoblock is set on a user's block, the autoblock would propagate when a user attempts to log in, and this setting would also prevent users who are not IP block exempt from logging in when set on an IP address.
This feature would be most useful in the GlobalBlocking extension if it was also possible to prevent local wiki administrators from overriding the global block.
Users would still retain the ability to download their data with this (at least on public sites). If a user blocked with this option logs in, below the block form the only options should be "Download your data", "Change password", and "Log out".
Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
The primary use case is global locking. Right now on WMF wikis it does not seem possible to configure both global locking and global autoblocking for a user. After this feature is implemented and it is possible to make a global block non-overrideable, existing global locks can be converted to global blocks, preserving their entries in the global block log, and the global lock feature can be deprecated and/or removed.
Another use case would be for cases where it may be needed to log out the user from a single wiki, mostly applicable to certain private wikis where automatic sign-on is disabled and requires approval to sign in.
Benefits (why should this be implemented?):
Implementing this feature is advantageous for single wikis and for wiki farms as discussed above.