[Koha] Block web crawlers Bots

[JITHIN N]

My server memory usage is increasing by some web search engine bots by
access of OPAC. I tried put

*robots.txt in *

on /usr/share/koha/opac/htdocs

As

*User-agent: **

*Disallow: /*

But still, my server is getting loaded by bots. The apache log file shows
bots names like petalbot, Googlebot, AhrefsBot etc. How can I block all
these bots' access to my OPAC pages?

With Regards

Jithin N

*System Information*

Koha 21.05

Deian 10