map $http_user_agent $uatype {
            default             'user';
            ~*(googlebot|bingbot) 'good_bot';
            ~*(nastybot|somebadscraper) 'bad_bot';
        }

You can also do something like this to rate limit instead of by IP address. Making all ‘bad_bots’ limited but not ‘good_bots’.

I’m not dismissing the difficulty of the problem but there are multiple vectors that can identify these ‘bad_bots’.