According to the docs, I could use - to exclude certain things from the search API... I tested it and this works on the language field for example, but doesn't work on topics.
According to the documentation:
note cats stars:>10 -language:javascript matches repositories with the word "cats" that have more than 10 stars but are not written in JavaScript.
But if I want to search for cats stars:>10 -topic:javascript it doesn't work anymore, although I tested it and there are exactly 9 repositories that have javascript as a topic.
So
cats stars:>10 returns 714 results
cats stars:>10 -topic:javascript returns 714 results (- doesn't work)
cats stars:>10 +topic:javascript returns 9 results
cats stars:>10 -language:javascript returns 602 results (- works as expected)
cats stars:>10 +language:javascript returns 112 results