Well you hire them so if you don't want them to teach, tell them not to. However, note that if the child spends a lot of time with this person, when they start talking they will pick up on the accent, just from hearing them speak a lot. It's not reasonable to expect the au pair not to speak
Personally I'd be happy she is working with kiddo and not just sitting on her phone all day.
My daughter who is 4 years old has a British accent right now from her mother. I wouldn't worry about it, as kids will pick up real quick how to speak once they start school (I.e., not daycare), interact with teachers and fellow students and it will fade.
I didn't speak any English until I was 8, and 1 in 100 people sometimes comment that I have a slight accent.
It's not something I would worry about long term.