Hey Greg O., one option you could do is to reset your duckling (Settings > Ducklings > Reset Duckling). This will stop all running queries though, not just the one you want to cancel.
Could you also try cancelling the query with MD_INTERRUPT_SERVER_CONNECTION? You can get the client connection id from md_active_server_connections()
SELECT md_interrupt_server_connection(<client_connection_id>);
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