I have been pestered with similar job referral requests by an
annoying interesting college acquaintance whom I didn't want to work with. I eventually figured out that the trick was not to respond with either no1 or yes, but to give an ambiguous reply to procrastinate the response forever, like this:
I will contact you in case I think I can refer you to a suitable job opening.
He still hounded me a couple of times more, but I was determined to stand my ground, and responded with:
As I said before, I will contact you if I think I can refer you to a suitable job opening.
I think we have talked about this a couple of times, I will contact you if I think I can refer you to a suitable job opening. If I do not contact you, you can assume that there are none. You don't need to ask me every week.
At this point, he either believed me (because I said it politely with a smile) or he "got" the idea, and did not bother me again.
You might be wondering if my response was dishonest, but it was not. I said I would contact him in case "I think I can refer him to a suitable job opening", which was
a blatant lie metaphorically true. The common language interpretation of that statement is that I would look for job openings in my company, shortlist any suitable ones that I can refer him to, and contact him then. However, because I didn't want to work with him, I didn't think I could refer him to any suitable job openings. :)
1 I usually respond to unwanted job referral requests with some variant of the "no, sorry" suggested in Lilienthal's answer and that is usually the end of it. This particular acquaintance, however, was a special snowflake, and only changed the form of pestering. He started annoying me with requests like, "No, you don't recognize my potential.", "Let's meet this weekend so that you can better appreciate what I bring to the table.", "Please give me feedback on what you think is missing.", etc.