Hi - I have had cats for several years and at the present I have 6 (3 neutered males and 3 spayed females - all indoor). I has been my experiece that if a cat sprays a lot then it is usually caused from stress which in turn can turn into a urinary tract infection and this is the cats way of telling you that he/she is in pain. In most cases the vet will put the cat on antibiotics for about 10 days. I can't imagine a vet suggesting that you have the cat 'put down' because the cat will always spray. I think you need to find another vet. Anyway, good luck and I hope it all turns out good for you!
carrollcats