The St Martin's Constables warn riders it's illegal and damaging paths.
The constables have posted pictures on social media of the damage done by thick mountain bike tyres on the cliffs at La Bouvee.
This is part of the area made famous by the book, The Guernsey Literary and Potato Peel Pie Society.
It is included in walking tours, inspired by the book and film.
But on a small island with very few challenging tracks for mountain bikers, some are using the cliffs.
It is illegal, as signs clearly indicate, and anyone caught could face a fine.
A trial, run by the Guernsey Velo Club over the past winter, allowed mountain bikers with a licence to ride a short section of cliffs at Pleinmont at night.
This is not the case anywhere on the cliffs in St Martins.
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