I had the same experience with alot of posts that I had not any interest in seeing. I would do the same clicking the x or reporting the post...all of the options. I think facebook algorithm was doing exactly what I didn't want to happen it showed me more of the group because I was clicking on the link. I stopped trying to delete the posts and would click on alot of other posts when they came up and it didn't take long and I was not longer seeing the post I had no interest in. Your mileage may vary.

Wally