Let say someone is brute forcing my EC2 website and i block that ip address using my EC2 ubuntu firewall and now if that user from that ip would access my website it will show 400 BAD REQUEST.
So my question is that will amazon charge me for this 400 BAD Request as amazon charges you for each request sent out of your instance.
All data coming is actually free, however the point in sending back data out of the network. If you blocked the IP address inside your instance itself in some kind of 400 HTTP response code as you shared in your question, that means the request will go all the way through their network until it reaches to your instance then you block and send back bad response back in this case it is like normal request with special kind of response I believe they will charge you in this case for any data transfer in/out of their network.
However, If you make such blocks in security groups or network access control list NACL you wouldn't have any charges or fees for such kind of data transfer.