WordPress delete mysql rows with string

Look at the documentation, $wpdb->delete() takes at least two parameters, the table name and the Where-conditions as an array(). LIKE isn’t supported for delete imho, so you’ll need to use ->query

global $wpdb;
$wpdb->query( "DELETE FROM $wpdb->postmeta WHERE meta_key LIKE '_cb_%'" );
$wpdb->query( "DELETE FROM $wpdb->posts WHERE post_type LIKE 'webcam'" );

I assume the double closing parentheses and missing quotes were just from copy pasting?