Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [name of babysitter],
We are sorry to say that we will not be requiring your babysitting services again in the future. We don't feel like you are the best sitter for our [child/children]. You don't seem to get along very well with them, and we are no longer comfortable leaving them in your care.
We would appreciate it if you would not contact us again regarding the need for a babysitter, and also please do not visit with our [child/children] when you see [him/her/them] out in public.
Thank you for respecting our wishes regarding this issue.