You could always try various counterattacks - returning a repeating compressed stream that decompresses to several terabytes, or returning one byte per second, or returning an endless list of hyperlinks to a honeypot, if the date is older than 200 years.
Sure, a lot of countermeasures is possible. I will definitely consider it next time, but for this calendar which had sitted unused for years on this instance, i decided to just turn off the plugin. A random wordpress plugin is never only a DoS avenue but also a bunch of potential vulns.
