My approach would be one of two things;
First, try talking to her. Politely explain the ettiquette. Invite her to join your lane if she will do your workout on your intervals. Otherwise suggest strongly that she find a lane that will suit her better.
Second, swim fly. This seems to deter a lot of people. Sounds like it might not work with her.
Yesterday I was swimming on my own before I coached. The pregnant ladies class was going on at the same time so there were only 2 lanes available. I got the dreaded wall lane. There was one swimmer in the other lane. A third swimmer came by and asked me if I could share. I quickly surmised that she would be a pain to share with and said sure but said that the non-wall lane is easier to share. She went into that lane just fine.