Frogs

Does anyone know how to best serve a notice to a frog? One of them has moved in rent free with the understanding that he would handle munching down mosquitoes and keeping the yard free of this pest. Well, the human has been eaten alive by them, and you won't believe how much having an ichy, miserable human slows down the service here. So, how can I serve notice to this frog to make him do the job he promised to do? Those mosquitoes must be removed!