how can i see previously banned peer ?

The peer is banned permanently, for all torrents, and even after restart. Basically, it is added to a blacklist.
Unfortunately, there is not yet any way in the UI to revert this manual blacklist.

You can however, find the blacklist in the following configuration file:
~/.config/qBittorrent/qBittorrent.conf

File the line starting with
IPFilter\BannedIPs=...

You can alter the list and restart qBittorrent to take changes into consideration.

For Bittorrent 3.3.9 on OS X I made a Applescript to unban peers

try
 display alert "Delete banned IP's?" message (do shell script "cat ~/.config/qBittorrent/qBittorrent.ini | grep 'BannedIPs=' | awk -F = '{print $NF}'") buttons {"Cancel", "OK"} cancel button 1
on error number ernb
 if ernb = -128 then return
 display alert "No banned IP's to delete."
 return
end try
if application "qbittorrent" is running then tell application "qbittorrent" to quit
repeat 150 times
 try
  delay 1
  do shell script "pgrep qbittorrent"
 on error
  delay 1
  do shell script "cat ~/.config/qBittorrent/qBittorrent.ini | grep -v 'BannedIPs=' > ~/.config/qBittorrent/qBittorrent.ini"
  return
 end try
end repeat
display alert "Could not quit qbittorrent"

Update or Applesscript to delet baaanned IP's:

try
 set var to do shell script "cat ~/.config/qBittorrent/qBittorrent.ini | grep 'BannedIPs=' | awk -F = '{print $NF}'"
 if var = "" then error
 display alert "Delete banned IP's?" message var buttons {"Cancel", "OK"} cancel button 1
on error number ernb
 if ernb = -128 then return
 display alert "No banned IP's to delete."
 return
end try
try
 tell application "qbittorrent" to quit
end try
repeat 150 times
 try
  delay 1
  do shell script "pgrep qbittorrent"
 on error
  delay 1
  do shell script "cat ~/.config/qBittorrent/qBittorrent.ini | grep -v 'BannedIPs=' > ~/.config/qBittorrent/qBittorrent.ini"
  return
 end try
end repeat
display alert "Could not quit qbittorrent"

Ok, finally the version that works on any circumstances for qBittorrent 3.3.9 or higher. OS X

try
 set var to do shell script "cat ~/.config/qBittorrent/qBittorrent.ini | grep 'BannedIPs=' | awk -F = '{print $NF}'"
 if var = "" then error
 display alert "Delete banned IP's?" message var buttons {"Cancel", "OK"} cancel button 1
on error number ernb
 if ernb = -128 then return
 display alert "No banned IP's."
 return
end try
tell application "qbittorrent" to quit
repeat 150 times
 try
  delay 1
  do shell script "pgrep qbittorrent"
 on error
  deleteIPs()
  return
 end try
end repeat
try
 do shell script "killall -c -9 qbittorrent"
end try
repeat 150 times
 try
  delay 1
  do shell script "pgrep qbittorrent"
 on error
  deleteIPs()
  return
 end try
end repeat
on deleteIPs()
 delay 1
 do shell script "cat ~/.config/qBittorrent/qBittorrent.ini | grep -v 'BannedIPs=' > ~/.config/qBittorrent/tmp ; mv -f ~/.config/qBittorrent/tmp ~/.config/qBittorrent/qBittorrent.ini"
end deleteIPs