Then just put the site under https://cloudflare.com/. It's free, and is excellent to help prevent DDOS attacks. It's not full proof of course, but it definitely helps, to the point it's a requirement for any new clients of mine.

I have my own sites under CloudFlare and can vouvh for it. Again, it's free. You just need to switch the domain's DNS over to CloudFlare's nameservers, and the site will be far better protected against bot attacks. Again, not full proof, but nothing is in the online security world. However, without question it helps greatly.