No Means No is a hard rule to argue with, but I suppose if you meet an agent at a conference you can try to convince them they did, in fact, request your manuscript. The chances of that working are pretty slim.
That leaves referrals.
I get a lot of queries from writers claiming they were referred to me. Sometimes they give a name of the person who referred me, and sometimes they do not. Of the "referrals" I receive, so far about five of them have been real. Because I'm an optimist and I love writers and I like giving people the benefit of the doubt, I choose to believe that the non-referrals were simply mistakes, and not a conscious attempt to trick me. After a few dozen non-referrals, I'm beginning to wonder whether some writers don't know what a referral is. That maybe, like "upmarket" and "high concept," it's a word that's been getting thrown around so much that people stopped trying to figure out what it means. Maybe.
A referral is a personal recommendation based on knowledge of an agent's taste, and more importantly, a personal relationship with the agent. That's the only time something can be called a referral. More often than not, the person who referred me to a writer will call or email me to say "Hey, I just sent someone your way." That way I can be on lookout for a query that I know will be tailored to my interests.
What's not a referral?
1) The editor or friend who referred me is someone I don't know.
Many of the non-referrals I get involve the name of an editor and the writer saying "________ is interested in my manuscript and suggested I contact you." Having interest from an editor is a big deal, and I appreciate when writers let me know about it. But I have to pause when they claim the editor suggested they contact me. Did this editor really say my name, or did the editor simply tell the writer to find an agent? If I don't know who the editor is or received confirmation from them, I have to assume it's the latter. Similarly, I get queries from writers saying a friend gave them my name. This is probably true, but again, who is the friend? If your friend read that I represent your genre and gives you my name, then that's good advice, but it's not a referral.
2) The writer offers a vaguely phrased, "I was referred to you" or "You came highly recommended," and doesn't say who did the referring.
In these cases, I fill in the blank and say "by the Internet." Sites like Writer's Digest, Agent Query, Query Tracker, and all the other curated lists of agents out there are great resources. Every writer should know them and use them. Just don't pretend a general reference list is the same as a personal reference. If I can't take your referral source out to lunch to discuss your manuscript, then you shouldn't mention it in your query.
3) The writer is querying me with a genre or topic I don't represent.
This one should be obvious.
Sometimes I think the word "referral" sets off alarms in writers' heads, like it's a secret code word they think they need to say to get their query noticed. Writers, you don't have to do this. We know you're lying as soon as your "referral" isn't backed up by facts, an actual human, or knowledge of what the agent represents. That reflects poorly on you as a professional, and could very well backfire even if your book is great. Agents want to work with writers we can trust and develop a good working relationship. It's a waste of your time to query the wrong agent for your work, and the right agent wouldn't need to be misled. We just need to love your book. No bells, whistles, or false claims attached.