What I would say to do is write IP's from your toolkit or what ever you are using for reading your access.log and those that trigger and spam the 503 error within milliseconds or what ever range it is you can do an API call and add those IP's to be blocked at a router level.

With CloudFlare you can have CloudFlare block those IP's before they reach your server like so https://api.cloudflare.com/#user-level-firewall-access-rule-properties

If you use OVH you can write the IP's that trigger 503's to OVH's Firewall https://api.ovh.com/console/#/ip/{ip}/firewall#POST

This should be of interest too https://twitter.com/olesovhcom/status/779297257199964160

But anything the firewall does not get your server now has a way to communicate with your router / firewall to prevent this requests even hitting the machine any more.

http://www.networkflare.com/