Currently the use of the Special:DisableAccount is not logged, not that I'm aware of. We only can guess that an account has been disabled because the 'inactive' userright suddenly appears on the disabled account, but that change of rights is not logged anywere.
I think that the use of this nuclear tool should be logged for transparency in the block log, in a new 'disabled accounts' log or, as CU, on Special:DisableAccount/Log.
Thanks.
Version: unspecified
Severity: enhancement
URL: http://www.mediawiki.org/wiki/Extension:DisableAccount