Based on Bug 13177 and Bug 22319 we now have working methods for disabling account access to the private wikis without deleteing accounts or password resets.
Currently this can be archived by using a usergroup with the view right or via wgBlockDisablesLogin, With the latter already being used on a few (see 22319).
There is a list of a few of the private wikis at Bug 13177#c8. Although there is no formal consenus around, due to nature of these wikis it should probably be done anyway.
CC'ing: Cary and Guillaume
Version: unspecified
Severity: enhancement