Headhunters contacting me is ok but sometimes, it's bothering. You have your own work and you are very busy. They contact you as if they would introduce a new opportunity and you wait. The effort of wait comes in vain in many cases. Many headhunters wouldn't bother to notify you even after it doesn't go well. You scratch your head with a sour feeling. The emotional burden is not much but if you can avoid, why not?
In addition, headhunters are known for resume hunting. (Reference) After you get no notice from many headhunters, you start to worry about the leakage of personal information.
I think it's important to know if the headhunter is serious with you as early as possible. If you notice that he was only for the resume, you would feel less anxious without much expectation. And you can choose not to send your resume to protect your personal information.
My question is, how do you know the headhunter is not serious with you?