Yes, you have a right to not allow the investigator to come to your clinic, until and unless there should be litigation and she (her company or her client, who would be the litigant) specifically gets a subpoena or other court order requiring you to provide them access--something which is very unlikely, but not actually impossible. Until there is a subpoena or court order, then you would only need to provide her access if you had in some way consented to it (e.g., you have probably consented to cooperate in your own insurance carrier's investigations).