Clear all crons in WordPress. This is a nuclear option, so use with care.

wp-clear-all-crons.php

// Pull all crons from options
$crons = get_option('cron');

// Kill each one.  Filter by timestamp or hook_name if desired.
foreach ($crons as $timestamp => $hook) {
    foreach ($hook as $hook_name => $details) {
        wp_unschedule_event( $timestamp, $hook_name );
    }
}