Hi,
To avoid that you have to remove the characteristic value itself.
You can also use another mode for the variant selection, not using dropdowns but using a table which will list all existing possibilities.
For "shape", depending if it can influence on the product quantity, the product price ; you might use a custom field instead of a variant.
The "item custom fields" is a feature of HikaShop Business, it allow the customer to put extra data while adding a product in the cart.
You can test the "item custom fields" thanks to our demo website.
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.