How to prevent values from changing when relative cells are changed?

[Gogpo2]

How can I stop values in column 1 from changing when column 1 has inputs (e.g. D2) from column 2 which I want to delete (hence erasing the value in D2 and subsequently changing the values in column 1)? See screenshots below for a clearer explanation:

Captura de pantalla 2015-04-10 17.03.09.png

Captura de pantalla 2015-04-10 17.03.20.png

Captura de pantalla 2015-04-10 17.03.35.png

How can I avoid getting "#REF!" and instead just have the original values in column 'C' as seen in the image below (and first image)?