frm_before_destroy_entry

add_action('frm_before_destroy_entry', 'my_custom_function');
function my_custom_function($entry_id) {
  $entry = FrmEntry::getOne($entry_id);
  if ( $entry->form_id == 5 ) { // change 5 to your form id
    // do something for entries deleted from this form
  }
}

As of v2.0.22, this is not longer needed. You can instead check the box in the field options for your file upload field.
Use this code to automatically delete file uploads when an entry is deleted. No changes are necessary.

add_action('frm_before_destroy_entry', 'delete_entry_uploads', 10, 2);
function delete_entry_uploads( $entry_id, $entry = false ) {
  if ( empty( $entry ) ) {
    $entry = FrmEntry::getOne( $entry_id );
  }
  if ( empty( $entry ) ) {
    // If there is no entry, don't continue checking for files
    return;
  }
  $upload_fields = FrmField::getAll(array('fi.type' => 'file', 'fi.form_id' => $entry->form_id));
  foreach ( $upload_fields as $upload_field ) {
    $media_id = FrmEntryMeta::get_entry_meta_by_field($entry->id, $upload_field->id);
    if ( !$media_id && $entry->post_id ) { //if this is a post field, get the value
      if ( !isset($upload_field->field_options['post_field']) ) {
        $upload_field->field_options['post_field'] = '';
      }
      if ( !isset($upload_field->field_options['custom_field']) ) {
        $upload_field->field_options['custom_field'] = '';
      }
      if ( $upload_field->field_options['post_field'] ) {
        $media_id = FrmProEntryMetaHelper::get_post_value(
          $entry->post_id, $upload_field->field_options['post_field'],
          $upload_field->field_options['custom_field'], array('type' => 'file')
        );
      }
    }
    
    if ( !$media_id ) {
      continue;
    }
    $media_id = maybe_unserialize($media_id);
    if ( is_numeric($media_id) ) {
      wp_delete_attachment($media_id, true); 
    } else if ( is_array($media_id) ) {
      foreach ( $media_id as $m ) {
        if ( is_numeric($m) ) {
          wp_delete_attachment($m, true);
        }
      }
    }
    unset($upload_field, $media_id);
  }
}

Use the code below to prevent entries from embedded forms and repeating sections from being deleted when the parent entry is deleted. This will automatically apply to all embedded forms and repeating sections in the form.

add_action( 'frm_before_destroy_entry', 'my_custom_function', 9 );
function my_custom_function( $entry_id ) {
	$entry = FrmEntry::getOne( $entry_id );
	  if ( in_array( $entry->form_id, array( 5, 6, 7 ) ) ) { // change 5, 6, and 7 to your form ids
	  	global $wpdb;
	  	$table = $wpdb->prefix . 'frm_items';
	  	$wpdb->update( $table, array( 'parent_item_id' => 0 ), array( 'parent_item_id' => $entry_id ) );
	  }
}

Use this code to delete a WordPress user from your site when their entry is deleted. Just replace 10 with your form ID and 25 with the ID of your userID field. Warning: This action cannot be undone.

add_action('frm_before_destroy_entry', 'delete_user_with_entry');
function delete_user_with_entry( $entry_id ) {
    $form_id = 10;// Replace 10 with the ID of your form
    $field_id = 25;//Replace 25 with the ID of your userID field
    $entry = FrmEntry::getOne( $entry_id, true );
    if ( $entry->form_id == $form_id ) {
        if ( isset( $entry->metas[ $field_id ] ) && $entry->metas[ $field_id ] ) {
            $user_id = $entry->metas[ $field_id ];
            wp_delete_user( $user_id );
        }
    }
}