Technically these agents have no listing and I believe you could bring some kind of action against them or file a complaint to their Board of Realtors and/or the Deptment of Real Estate.
They could be slapped with a fine or have suspension of their license.
Good luck with selling your home.
That is not to excuse everyone in the real estate profession. There are some agencies taking advantage. In the example I described below, the information source turned out to be an automated daily bulk loading system from an Agency database direct to the Zillow database, hence Zillow itself was unable to remove it manually as you might expect. The complaint had to go from Zillow back to the agency for them to remove it manually from their upload. This is what took two weeks. I even sent an inquiry through the listing contact, and did not receive a response. At best, somebody in that agency made a mistake on an MLS listing number and claimed one that was not theirs, then there was nobody to field the inquiry at the other end. Not receiving any response is also damaging, because that person is then discouraged from inquiring about the same property again from a genuine listing ad, since there was no response from the attempt. This is as much of a concern as inaccurate information.
This is why, if you can identify who uploaded the information in the first place - whether your former listing agent, bulk upload load, or aggregator, then you can contact them directly and ask them to remove it. However, since often it is not possible to identify who did the upload, and since responsibility lies with the site that is hosting the information, you can only hold their feet to the fire. This is why I suggested it might be worth sending a bulk mailing of Cease and Desist letters, although it is only a suggestion, I do not know if this would actually work.
The unfortune part; because they are not a true MLS with all the bells and whistles, they are not maintaining up to the minute information. I have found properties of mine that are sold and closed and new owners living in, still listed as an old number. I did not put them on and have no way of taking them off.
These websites sometimes give the impression that other Realtors are the listing agents. I have had consumers call me very angry that I am "listing" their property and it is not with a Realtor.
So far as offenders such as Trulia, Yahoo, etc. it's not so easy in my experience, unless the ads were loaded by your former listing agent. If so, you need to instruct the agent to remove them and they would be legally obligated to do that for you.
Our problem was that we found another (big brand) agency had claimed one of our listings on Zillow, but no agent name attached, only the company branding. It took us weeks to get them to replace the agency link, and for almost a week of that they had our agency name next to their agency logo. If it wasn't so frustrating, it would have been funny.
You are absolutely correct to want the clean slate, and as soon as possible. You have to persevere, but I could not recommend our methods when it comes to the Zillows of this world, which was basically to follow their instructions. It proved extremely time consuming and I think if I ever found multiple offenders of the kind you describe, I might be tempted to tackle it with a shotgun blast of "Cease and Desist" letters, sent by registered mail to the physical contact addresses for each of the offenders. If you're not sure how to write one of those kinds of business letters, ask your attorney for a sample, or to write it for you.
Some of these sites think that "real world" rules don't apply in cyberspace and I think it needs something like that from the "real world" to get their attention. I can't say I have tried it, but it has to be quicker and less stressful that tackling them one by one, which could take weeks, and in the meantime others may be replicating the ads.
I would also make sure that your former listing agent has removed it from their site as it would not be uncommon for it to be left there for a while after a listing has expired if it is not re-listed with another agent.