You’re doing nothing wrong.
You used to be able to hard-eliminate a search term by putting a minus sign before it.
Now, the Google search engine does still pay some attention to that, but treats the minus sign as a suggestion. Basically, Google decided that their search engine is smarter than people who think they want to be excluding some search results.
While the specifics can change over time, the last time I checked, Google was not offering any way to override their search engine’s calculations on when it wants to show you information anyway because it determined it may really actually be able to pleasingly outsmart whomever made the request.