How can I overwrite or remove the values from a custom field?

It is not possible to set custom field values back to 'blank' without a 4Matrix Support Agent intervening.

If you try to import data for your custom field with nothing in the corresponding cells in your spreadsheet, these are ignored by 4Matrix and the original values will remain. However, it is possible to use a '/' symbol, '0' or 'null' if you require a quick fix.