Dear Mary: How do we stop our friend letting herself in to our house?

Q. Our friend lives far away, but comes to stay at her country place nearby several times a year. Some time ago, before she had wifi, we gave her the code for the rear entrance to our house so she could come in and connect to the internet. She made good use of this, and, in addition, gave the code to guests of hers who also appeared without a by-your-leave. This, we hoped, would stop when our friend got her own wifi connection, and indeed it did. However, her unannounced visits have continued, to the extent that she still just walks into the dining room when we happen to be eating, and sits down at the table, or comes into the sitting rooms, or wanders into the garden if it’s where we’re to be found. Changing the code means she asks for the new one, and now that a fault which no electrician seems to be able to repair means we cannot change it again, we appear to be stuck.