Let me try to make it seem better than it sounds:
Would you say that a shadchan should only bother calling with a shidduch when it is absolutely certain that it will work (say, that at least 5 dates will come of it)? No way. If shadchanim limited themselves so much, that would cut the dating arena to a tiny fraction of what it is. Obviously, it is worthwhile for a shadchan to have lower standards than that, in order to keep things moving (which leads to a better understanding of what each person wants, and leads to more successful shidduchim). Find out from your “experienced” shadchanim what their success ratio is. Ask them if they think that they could have succeeded without the failures.
I think everyone agrees with what I just wrote above. Now to the issue at hand.
Should a shadchan take no for an answer easily? I think not. If they did, only a tiny fraction of dates would happen. Therefore, they must push and push until they are convinced that it is not worthwhile.
That means that a person who is dating will find himself in a position of frustration many times. Both because many suggestions are not worth looking pursuing and because the shadchan doesn’t want to accept a “no” easily. Frustration is necessary in order for the process to work. I’m sorry (and so is the shadchan) that it seems like you are being harrassed, having your time wasted, and not being taken at your word. But if you agree with what I wrote you will see why it must be this way.
My advice is, if you want to minimize your frustration – give a shadchan (or more than one) a chance to get to know you. After explaining why when you say A you mean A, you begin to develop a relationship with the shadchan (hopefully it will be short-lived!) and the shadchan will learn to understand you and trust you. To someone who trusts you, “no” does mean no.