Hello,
Since you have one variant with "0.26" value, that value is available in the dropdown to allow every combinations.
They are thread in that forum asking for the same feature ; but you current request is not "possible" since "Carati" is in second position.
Because you have "Bianco - 0.26 - 10", when the customer select Bianco, it can choose "0.26" but once these two are selected, if we follow the logic of the feature ; the misura "16" wouldn't be available.
Such feature require specific and custom JavaScript development in order to interact with the dropdown to disable not available values.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.