There is a slim chance he's just unavailable,
Maybe, but if you check the "Untrusted feedback" part of his/her "Trust summary", you'll see a lot of complaints against the user in question.
How do I leave a negative feedback on his profile?
Go to his/her profile > Click "
Trust" > Check the type of rating that you want to leave, then add reference link and your comment > Click "Submit".
Or I can't, because I'm only a Jr.Member?
You can, but you'll be the only one to see it (unless you're a DT1/2 member).