"Junk leads" usually come from shared platforms where you are competing with five other agents for a lukewarm prospect. To stop buying these, you must become your own lead source. This is done through branding.
When you provide specific, valuable information: like a guide to the best school districts or a breakdown of local property taxes: you attract people who are actually interested in your expertise. You aren't just a name on a list; you are a solution to their problem.