AFAIR, No they can't. They can only block you from sending them emails.
Perhaps you are searching for her by her last known name, and she may have changed her name.
Easiest thing to do is in your emails.... whether in Inbox or Sent Items....., select "ALL" from the top, so that your emails are now grouped by Date ONLY, sent items are mixed with Inbox items.
Then select "view profile" from the email that you sent her, and you automatically go to to her profile. This then tells you what her current username is.
TIP... If you end up sending emails to any profiles, I would suggest that you add her to your HL, or a subset of it, that way you don't have to constantly search for her, as she should be in your HL.