procedure clearField(fieldno : Integer);

The function clearField() removes all data from a certain field and fills it with zeroes (0x00) or spaces (0x20).
You can call this function before calling setField(), setMemoBuf().

begin
    // your code
    dbf.clearField(0);
    dbf.setField(0, 'some value', 0);
    dbf.writeField(row, 0);
    // your code
end;