<%= link_to 'Destroy', post_comment_path(@post, comment),
data: {:confirm => 'Are you sure?'}, :method => :delete %>
в комментариях контроллера:
def destroy
@post = Post.find(params[:post_id])
@comment = Comment.find(params[:id])
@comment.destroy
respond_to do |format|
format.html { redirect_to post_comments_path(@post) }
format.xml { head :ok }
end
end