We trap any feral cats (local pet store will rent one for $6/day or the humane society will rent you one) and then take them to the humane society. They will test them for disease (FIV/FIP/FELV). If they are positive, they likely will be euthanized. But better that than they die in agony of these diseases while going around infecting other cats. If they are clear of disease, they will be given a chance to be tamed and adopted. I can speak for the Pasadena Humane Society in that they give a cat every chance to be adopted. You can also agree to be the cat's "last resort", whereby when they have been there a while and they are on the short list for euthanasia, you can go and pick them up. You probably have to pay for the spay/neuter but it's cheap compared to taking them to the vet.
Good luck and hope you don't catch skunks!