Create a re-occuring SQL event in your phpmyadmin…
Change the database names in the example below to your database…
DELETE FROM
wp_xxxxxxx_sitemetaWHERE
meta_keyLIKE "%_site_transient_timeout_aiowps_captcha%";
DELETE FROM
wp_xxxxxxx_sitemetaWHERE
meta_keyLIKE "%_site_transient_aiowps_captcha_string_info%";
These are 2 separate rules that I have running every 30 minutes of every day.