I too, had this problem but a bit different. Somehow a hospital and pharmacy had our phone number as a doctors office. I didn't want to endanger anyone so would politely tell them that this was a home not a doctor. After the second call I researched the doctor and called to tell them what was going on. I also kept a note of the correct number next to my phone. They eventually got it straightened out.
I agree with sending the nursery a letter. After you do that, you can let them know that you might not have the patience to be nice to their potential customers anymore.