This neighbor can be sued for these actions.  I would have an attorney write up a little letter warning her to not interfere with the sale of this home.

I was hosting an open house (owner has been hospitalized for 3 months) and the neighbor parked right next to the driveway and made a large sign and posted it in the back of his pick up truck and added balloons. The sign said "Ask about the condition of the landscape wall". 

Try to call their bluff.  When he/she starts up, ask them to join you and show the point out the poor condition of the home.  Or, state what you said above,  to the neighbor, and ask the person to stop harassing your clients.  Advise them that if this causes the price to go down, their home value goes down too....sometimes bullies need to be confronted..good luck.  Never fun!

Yes, I think we all have. When I encounter such situations, my security system, Mr Sasquash, will intercept the neighbor at the end of the neighbors driveway until the buyer has escapted.

I find this issue also when a FSBO is down the street. MR or MRS DIY will give themselves permission to invite the buyer to take a look at their home before they leave. Sasquash has a way of prevailing on this predation upon the efforts of others.

I've been fortunate. I'm very surprised that, although I'm not free from this experience, in the few occasions I have had this happen it was of very little consequence.

YES, I work hard to keep any of my rental prospects from arriving before I do or the neighbor will be out in the driveway evaluating them and driving them away.

Oh, Yes,  We had one neighbor that wanted to buy the home next door at a low price, so he showed up during every showing and talked about how bad the property was, so they would not buy it and that the seller would keep lowering his price.  I do't think it worked.

That is awful, I hope the seller disclosed this pesky neighbor on their disclosure...

That is too bad. I would warn all agents showing the property to not listen and perhaps have a talk with the neighbor. Let all her venom come out and then maybe she will relax and be glad she is getting a new neighbor and hopefully a great one.

They're everywhere and welcome all the time Tamra. A couple of years ago I had a visit from one which earned me a $6M plus listing. We call that killing two birds with one stone. A 25 minute conversation sealed the deal, that's nice!

So, why don't you go outside and have a talk with that neighbor and lay your cards on the table?

Oh, yes. I recently had an open house of unfurnished condo. The kids of the family who live above got too active, and some OH visitors left in a hurry.

Prospects need to be warned in advance to avoid conversations with this neighbor. The showing agent should take control of the situation.

Couple things might work 1. fake buyer stops in for showing with their 11+ children (find someone with a daycare and bring them all to the house- just let them run around in front yard tell the neighbor they ADORE the house and might be next neighbor  2. Bring a noisy garage band with some real crazy looking cars to a showing and have them chat with neighbor about band practice every night 3. have someone else (not you or seller) video tape the neighbor  4. start sweet talking the neighbor - give them the promise of some rewards ahead if they play nice sugar works best. I would not go with Gabe's idea about legal letter, yet.  Don't threaten anything  unless you are ready to do it (legal fees could eat you alive)  Find out WHY they dislike the neighbor

oh yes and I quickly asked them to leave. It's my show my rules!

You are correct no body wants a neighbor like this.  I would speak with her and say it is not her place to speak with buyers who are seeing the home next door. As she is interferring with the sell of the home.  Ask her to please stop.

I kinda one time had the opposite....one time I can recall the nextdoor neighbor to the seller  ( who was a past client of mine)  came over the seller  went APE SHIT on me,,,, She never told me that was why she was moving.  The seller hated her neighbor.  It was awful.. But actually that seller  was a bit Cray cray anyway!!

Yes and it was hell until I unloaded on him. Then it stopped.