How can i lock a column in calc so that i can only write unique values?
There’s no such option in Calc. You may use formulas to easily identify when entered value already exists above (using another column, or conditional formatting), but not prevent the entry. There’s a Validity tool, but AFAIK there’s no way to use it to validate that there’s no such value in a range.
You may want to use Base for that possibly? That’s actually database feature.