DOG MESS

We own a CL on the Devon/Cornwall border.  We are a working farm with free range animals, children and pets.  It is a CC rule that dogs are kept on a lead and that mess is picked up.  I state this when  people book and if I think they will not follow this rule, then I suggest they book elsewhere, where dogs can run free on site.  I currently have 2 vans on site with dogs,  and according to other visitors on site, they are not picking up after them.  What more can I do?  I've put a sign up too.  We have regular visitors with dogs who come year after year who keep their dogs on leads and pick up so if I say we do not permit dogs, I will loose alot of my regular visitors.  HELP.