Have Cat Questions? Ask a Cat Vet Online.
My name is ***** ***** I have been a Vet tech for 30+ years with interest in Feline heath. There is no way to humanely Euthanzie the Cats. They are sensitive to most things and can cause horrible deaths. What we suggest is to trap the Cats and take the to a shelter where they would be neutered and spayed with vaccinations. They will mark the ear and either adopt, the cats out, Return to a Ferral community once neutered and spayed, or euthanize the ones that are ill or cannot be worked with in one of the other ways. There is a Program called Love that offers free and low cost spay and neuter: http://www.lovethatcat.com/spayneuter.html
There are Animal shelters and Animal Control, that also offers the same service. I am sorry but nobody on the site can tell you how to Euthanize the colony, and it it also considered Animal Abuse to do that.
I am sorry if you would like me to see if I can find an animal rescue group to assist in this situation, I can do some searches for you. Legally poisoning the cats or putting them to sleep with any type of medication is considered Animal abuse and punishable by law. There have been a lot about this in the news and is now punishable with jail time if caught.