A new variable, the autoclean_novehicles was introduced in the version 0.7.0, which specifies the amount of months after which an inactive company without vehicles is removed, regardless of the password setting. This is to prevent players from creating 'empty' companies.
For example if you set autoclean_protected to 36 and autoclean_unprotected to 1, password protected companies without activity will be unprotected after three years, all unprotected companies will be destroyed the first next month giving only a little opportunity to steal another player's company and scores. If you keep hi-score statistics of players, this setting might come handy to use. Also beware, that one game can't have more companies of the exact same name even if the older ceased to exist.