Maisonentravaux on "[Plugin: SendPress Newsletters] Sendpress and wp_postmeta : MySQL problems"

I applied these requests to clean all the lines SendPress generated in wp_postmeta :

DELETE FROM wp_postmeta WHERE meta_key = '_firstname_label ';
DELETE FROM wp_postmeta WHERE meta_key = '_lastname_label ';
DELETE FROM wp_postmeta WHERE meta_key = '_email_label ';
DELETE FROM wp_postmeta WHERE meta_key = '_button_label ';
DELETE FROM wp_postmeta WHERE meta_key = '_list_label ';
DELETE FROM wp_postmeta WHERE meta_key = '_lists_checked ';
DELETE FROM wp_postmeta WHERE meta_key = '_thankyou_message ';
DELETE FROM wp_postmeta WHERE meta_key = '_thankyou_page ';
DELETE FROM wp_postmeta WHERE meta_key = '_setting_type ';
DELETE FROM wp_postmeta WHERE meta_key = '_form_type ';
DELETE FROM wp_postmeta WHERE meta_key = '_sp_settings_id ';
DELETE FROM wp_postmeta WHERE meta_key = '_form_description ';
DELETE FROM wp_postmeta WHERE meta_key = '_collect_firstname ';
DELETE FROM wp_postmeta WHERE meta_key = '_collect_lastname ';
DELETE FROM wp_postmeta WHERE meta_key = '_display_labels_inside_fields ';

It seems to work but how to stop the injections ?