I can’t give you legal advice about a no-contact order, but I think there are a number of complications with going that route — not the least of which is the fact that there has been no contact with you directly and what your mom says the therapist is saying may not be accurate or even happening. Also, your mother talking about you is her choice — and that is what is important to her. As one-sided as it is, it is your mom’s therapy. All that being said, I would deal with matter more directly.
The most direct way is to ask your mom to be invited into a single session. Explain to the therapist what is happening — and that it is not welcome — and that it is actually reinforcing a one-sided perspective that isn’t healthy for you.
You don’t need to use a lot of force with this. Just taking the time to ask your mom to be invited in can have a profound effect.
Wishing you patience and peace,
Proof Positive Blog @ PsychCentral