The relatively new option to re-invite people who've previously declined to review a paper from within the referee list is great, but unfortunately it doesn't stop Editors from adding the person again from the search list - they don't always check the list of previously invited people properly, especially if there are a lot! Would it be possible for the system to run a check on this during the search (similar to the one it already does for checking whether a person in the search results list is one of the authors or from the same institution - see screenshot)? Then the alert box for those previously invited people could say "This person has previously declined to review this paper but can be re-invited from the referee list above”
