I sent a message to Nitroflare yesterday and got a reply back this morning.

Hello, is there a way to turn off the "I'm not a robot" verification when downloading files with Jdownloader? I sometimes have to check the box 6 times during the download for a single file.

Hello,

Due to increase security on our site following DDOS attacks,
you will be prompted with captcha on the download manager,
Please solve the captcha in order to use the manager or try using a different download manager.
In either case our service is working properly on the web browsers as well, you can try downloading from there too.