mod_sec - best way to ID user agents to block?

Hi Forum

I am new here so greetings to all !

I'm working on the best way to identify and stop bad user agents. I've read multiple threads and googled a lot - so far it seems there is no "clear" or obvious best practice so to speak.

At the moment I've implemented a mod_sec rule to include a txt file with a list of known bad bots etc.... but looking at logs of various user accounts on the server there appear to be a lot of hits from unknown spiders and bots - so how do you mitigate these in the txt file if you don't have a user agent name? What would be the best way to target "unknown bots"

Also - where would one find a list of bad user agents to block?

Thank you