IP Blocking does not prevent access to your forum. It blocks anyone using that IP from Registering, Posting, or Sending Private messages in your forum.
Baidu is a Chinese search engine and is bigger than Google in Asia and as China has the biggest population in the world, preventing Baidu from indexing your website could stop allot of people finding your website.
As Baidu is a legitimate serach engine like Google, Bing, or Yahoo, they do respect robot.txt files, so if you want to stop Baiduspider indexing your website create a robot.txt file and place it in your websites root with the following:-
#Baiduspider User-agent: Baiduspider Disallow: /
#Others User-agent: * Disallow: /
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