How to make affiliate link not visible to users?

Hi there!

I know this was possible in the old non-responsive theme and it was very easy to do. There was a field in the backend where I could paste affiliate link and no matter what users done in the frontend with the link it was always redirecting using my link.

So question is how to get this to work?

I know that I can replace user link in the backend but if I will have clever user, he will edit this post and past his affiliate link again, so I want to prevent this.

Best regards.