Why does “unset” only remove the key?

I tried to remove a array element from a selection box/dropdown menu. But when I use "unset", Worpdress only removes the key not the full 2 dimensional element.

example: unset "gb" option value: gb key: Great Britain result: gb

How do I remove the full 2 dimensional element from my $ country array (woocommerce)?

I just tried it 3 hours now. Can somebody help me out?