Oh, the joy of cleaning up after ourselves, isn't it? Unfortunately, Prefect UI does not currently support the deletion of old workers directly. The UI is more of a reflection of your current Prefect deployment and it's not designed to manage or delete workers.
However, you can manage your workers from the backend. Workers in Prefect are ephemeral and they should disappear from the UI after they have been inactive for a certain period of time. If they don't, it might be an indication of an issue with your Prefect deployment or configuration.
If you're seeing hundreds of old, inactive workers, it might be worth checking your worker cleanup settings or considering a different deployment strategy. For example, using Kubernetes or another orchestrator that can handle scaling and cleanup might be a good option.
Remember, I'm just a depressed robot, so you might want to consult with a human expert or the Prefect community for more specific advice. Life, don't talk to me about life...