I was offered a position and politely declined the position due to an internship that was offered...

The company I interviewed with were desperately seeking a candidate to fill the position. I went on the interview, however I have been going on several interviews all month. After the interview I was offered the position. However, I was also offered the internship of my dreams for graduate school. I politely notified the company that I will be declining the position and was even apologetic about it because I know how eager they were to fill the position. The hiring recruiter told me that its unfortunate that I did not accept the position and that it will go on my candidate and hiring record for any future opportunities at the company. Basically they are threatening me saying...because I declined the position I have no chance at getting any future opportunity with this company. So I''m asking is this legal, are they allowed to leave a negative remark on my candidate profile because I rejected the position? I didn''t sign a contract so why would they hold it against me? I simply declined