Sorry Twenty, but I feel that I have to take issue with your answer. Rarely are things that black and white in civil law - otherwise there would be far less work for lawyers.
As usual the answer (unhelpfully) is 'it depends', as this post made in a very good legal website, that I have recommended to AB before, makes clear.
http://www.swarb.co.u...dGateRightofWay.shtml
The following case, taken from the useful website Garden Law may also assist.
http://www.gardenlaw....topic.php?f=8&t=13205
Related to this question, for starters, there is no such thing as an 'open right of way'. If by this you mean a public right of way - that is a path open to everyone, then it is correct that the owner of the land cannot put a locked gate across it. However public rights of way can have allowable obstacles across them - stiles to control the movement of stock between 2 fields, for example.
If you mean a private right of way or easement - that is a right that applies to you as owner of your land, giving you as owner of that land (and others on your authority working as your agent) a right to pass and repass across your neighbours land, the locked gate arrangement may be acceptable in law provided you have a key and access is freely available from both sides using that key.
Often it is in your interests as well as that of the owner to have the gate locked and accessed only by a key that both of you use, to prevent others getting through.
What is your issue with them doing this?
Forget the conflict of interest argument - its a dead duck.