Many (uninformed) people think recruiters succeed by poaching, or persuading, people to leave their jobs for another. This could not be further from the truth.
The fact is we are successful when we solve problems for our candidates and clients. Meaning, that person’s manager has either failed to understand them or simply cannot help them, and a job change is inevitable. This simple point is profoundly important because we cannot convince anyone to do what they are not inclined to do. We can simply help them resolve an issue. It may be a bad manager, unstable company, poor commute, uncompetitive compensation, lack of growth opportunity, or a plethora of other reasons.
The takeaway is that managers can armor their teams against recruiters by being attentive to their people. Solve their problem before a recruiter does.