I suppose you will need to query DB for that (this is pseudo code)..

1) collect all user ids who were loggedin just before your time window of interest

SELECT [intUserID] FROM [dbEPiCoopCommon].[dbo].[tblEPiServerCommunityOnlineStatus] WHERE datLoginDate < '2012-11-28'

2) Loop through and remove those

collectionOfIds.ForEach(id =>
{
    CommunitySystem.CurrentContext.DefaultSecurity.RemoveUser(
        CommunitySystem.CurrentContext.DefaultSecurity.GetUser(id));
})

You will need to do it on regular basis for sure to remove proper inactive users.