Manual:$wgBlockDisablesLogin/en

Details
If set to true, all blocked users will be signed out and future attempts to sign in will fail. At runtime, any extra rights blocked users might have are removed as well (which is relevant when the blocked user has an already existing session).

Note
When using this with a public wiki, or a wiki that allows unregistered editing, the effect of logging out registered blocked users may actually be adverse. Unless the user's IP address is also blocked (whether directly or via autoblock), logging the user out will again allow reading and editing as an IP address. Unless the user's IP address is also blocked (whether directly or via autoblock), logging the user out will again allow reading and editing as an IP address.

Alternative
An alternative to this setting might be using $Autopromote with $blocked to create a group that revokes certain rights.