nothing but kde

Does anyone know which file I have to edit in to order to stop people logging into anything other than kde? Ive done it once before but I cant remember which file need editing. If you need more information just ask as I dont feel Ive been very clear, but cant think of any btter way to put it Cheers -- Andrew Nix St Pauls Catholic High School Firbank Road Newall Green Wythenshawe Manchester M23 2YS tel: 0161 4922040 fax: 0161 4982030