Posters and fairy doors, set up as part of a scheme to reduce anti-social behaviour, have been removed from Burton Riggs Nature Reserve.
The items were taken from the reserve and scattered across nearby rail tracks.
The Lifestyle Challenge scheme was set up by police and local school girls, in a bid to reduce dog fouling and anti-social behaviour in the area.
The girls, aged 11, arranged a cake sale to fund the production of the posters aimed and bought the fairy doors to create a fairy hunt at the reserve.
Sergeant Neil Northend from North Yorkshire Police Partnership Hub said: "It is really disappointing to see the project that the team had spent so much time and effort on be vandalised.
“This is the reason why we encourage young people from across the county to be involved with Lifestyle Challenge, to give them focus over the summer, to make a difference in their community, and the girls did just that.
“The fact that someone has had a complete lack of regard for the girls’ selfless work is totally unacceptable”.