How to delete content of sheetnode spreadsheet field?
Solution I found was to not save the data in the first place. Will make debugging harder, but here it is: 

function hottopicsresearchsheet_node_presave($node) {
  // Check that the nodes of the type I need and process data...

  // Clear data.
  if (!empty($node-&gt;field_spreadsheet[$node-&gt;language][0]['value'])) {
    unset($node-&gt;field_spreadsheet[$node-&gt;language]);
  }
}