Recruitment agency representatives, or so-called headhunters, earn their money by finding employees companies can't find for themselves. Thus they work primarily for companies, not for job seekers. They can only find jobs for those whose qualifications and skills match the requirements of the companies who are their clients.
All that is fine providing the headhunters treat candidates for the vacant positions professionally and with respect. But as almost anybody can become a headhunter, many of them are totally unable to behave professionally. If you communicate with candidates in the following ways, it is understandable that they will want nothing to do with you.