You might be in a position of being a landlord in this situation, but it's not a question of getting the roommate to leave.
When you say roommate, do you mean a co-tenant, someone who was also on the lease? Did you pay rent together, i.e., you paid half to the LL and she paid half?
Or was she someone who came on as a real roommate - i.e., occupant - who you brought in after you took occupancy, and you're the only person on the lease? If so, how was the deposit handled?
Which of these situations best describes your situation? Please give us more details as to how things worked.
BTW, for now ignore the lawyer.
As for her getting into the building, you can change the locks to your apartment. You can tell the super you suspect she made copies (even if she returns the keys) and he might wish to change locks for the building. But if it's a large building they might not do that.