Great question, Carol. My understanding is you cannot prevent your comments from showing in your stories feed. It seems this has been an issue going back two years on Medium.

I wrote about two potential solutions to solve the "comment-as-a-story" problem. One is a chrome extension a Medium user invented, and another solution is utilizing the stats page.

I highlight both solutions:

https://medium.com/fellowship-writers/how-i-separate-my-comments-from-my-stories-56ac5a7378e7

Hope this helps!!