Thank you Remirus.
I've tried what you told me by resetting peer information and setting predefined hosts as follows:
-Server-A: Disable relay and tracker setting. Only Server-B as predefined host
-Server-B: Disable relay and tracker setting. No predefined host.
-PC-A and PC-B: Disable relay and tracker setting. Only Server-B as predefined host
First I put Server-A, PC-A, PC-B online, and I see that they don't connect to each other, which is good. The problem comes when I put Server-B online. As soon as I put Server-B online, all peers are discovered even on PC-A and PC-B, which makes them start downloading from both Server-A and Server-B. It seems that Server-B, which is working as an intermediate, is sharing the peer information to all peers.
Is there anything else I should be doing?
Thanks again.