How do i stop my cat spraying ?

My 3 year old male cat keeps spraying everywhere in my house! He has been neutered (when he was 6 months old) but he stil does it all the time. Sometimes my other boy cat, also 3, sparys too, and occasionaly my female cat wil join in (i have actually seen her spray for people that say its not possible) Ive had them all spayed/neutered so i dont know what to do now. I tried citronella spray but it didnt work. Any one have any ideas (that dont involve guns!) As its getting a bit much, he sprays on my cooker, my kitchen tops, my wardrobe, my hoover, tv, speakers, just everywhere!!!! Help!