Rest API: Remove value(s) from a customfield

When a customfield can specify multiple values and cannot be empty, I can't remove old value(s) when I want to update field.

For example, non-empty field FieldName has 3 values: A, B, C. I want to change it to just A.

Command "FieldName A" has no effect. And I can't find a way to remove certain values or all values from the field.

Hope you can help me. Thanks in advance.