There is no legal case here

What you have is normal course of business.  You asked for people's contact for new jobs.  Those people provided new job leads to you.  You got offended because the jobs they provided is too insulting, that's your problem.   They did not harass you as harassment is defined in legal terms.  That is my legal advice to you. 

Tell them you don't want to hear from them again.  Why do you have to use f-word or cease and desist letter?  Can you not talk to people in a calm conversation?  I thought you were managieral material. You certainly doesn't behave like one.