After deleting the description file by mac OS, it still shows that nextdns is used

I tried nextdns by installing the description file, but after finding a lot of problems, I deleted the description file. It turned out that even if I successfully deleted, the configuration dns displayed in the nextdns interface was still using, and it caused some of my apps to fail to connect to the server every time I recovered from sleep mode. Every time I need to exit the app and restart it before I can connect.
I don't know why it still works after deleting the description file. Why is it still not deleted?

And: Does this product have no customer service contact information? Can it be solved only through the community? 
I looked at a service product, and there are so many feedbacks on the community? It surprised me.

So after installing and deleting it through the description file, what do you need to do to completely uninstall and not use nextdns?
Is the product only responsible for installation and not uninstallation?