How to remove the sidebar on a blogger in the Soho theme

Go to Theme > Edit HTML, click somewhere on the code window and do a search for this:

  .page_body {
    margin-$startSide: $(sidebar.width);
  }
  .sticky .centered-top {
    padding-$startSide: $(sidebar.width);
  }

Then delete them ! That should do it. Note that this might not apply to other blog themes.