Thanks for your question.
That is a very unpleasant situation! Very difficult to prevent this from happening.
Have the agent attend with the viewers or have them set up block viewing. Have a number of people attend at once with an agent or two present to ensure that the neighbour cannot do this. If the agent wants to sell it will have to assist to achieve this.
Alternatively, think about putting it in auction. They tend to get less viewings. You can set a reserve price at auction.
It is difficult to stop your neighbours but hopefully your agent can assist to minimise this.