my ymail merges with my gmail account

I have created an ymail address because my gmail has been hacked and most emails destroyed. The hacker must be an obsessive maniac because every ymail now shows also in my gmail inbox (after the email title, it says via yahoo).
How can I avoid forever this occurrence? I do not want to merge them (or I wouldn't have a second email address!)
can someone help please?
It is not the only issue I have with my computers, but I do hope to solve at least this one

I have managed to delete an alternate email address and any emails from Google + from a public computer! I hope this will solve the problem. Thank you