Do you have a copy of the old permissive access route, which gives a date on it, as to when it expired. You could put that up alongside the sign explaining there WAS a path, but it was only temporary and has been closed since 2011, as per the council notice.
We had to do that with a permissive access we had across the farm, which actually shut 4 years ago. 2 years ago we had an impromptu camp in one of our lower fields and these 2 ladies started arguing with my OH about being able to walk through our campsite. It didnt seem to matter that he told them he owned the whole farm, they didnt want to get off and started arguing 'right to roam' at him.....
10 mins later it was a woman and child ON HORSEBACK!!
And then a bloke with a 4x4 and trailer with a kids quad bike on the back, and it went on
I dont think we had been aware about how much people were using our bottom fields without us knnowing until we stayed there for the weekend.
We found it very irritating that the ramblers and the council were very keen to advertise when new paths were open, but not quite so keen to inform once the permissive ROW were closed.