Manual:$wgBlockDisablesLogin

Details
If true, blocked users will not be allowed to login. When using this with a public wiki, the effect of logging out blocked users may actually be adverse; unless the user's IP address is also blocked (e.g. autoblock), logging the user out will again allow reading and editing, just as for anonymous visitors.

An alternative to this setting might be using $wgAutopromote with APCOND_BLOCKED to create a group that revokes certain rights.