Did you actually read their email to you? You have console access so you should probably access your droplet that way, find how your droplet got compromised and

Did you actually read their email to you? You have console access so you should probably access your droplet that way, find how your droplet got compromised and remove what's causing the outbound DDOS. DO is an unmanaged service, so if you're incapable of managing and securing your server, perhaps try a managed provider.