Re: Killing comfrey I'm reading a book called "Organic gardening" by Lawrence D Hills (1977). Its very good for vegie gardening. He talks about growing comfrey for liquid manure, and along with lots of other info on the topic, he says "If necesarry a comfrey bed can be killed with a solution of ammonium sulphamate at 1 lb to 1 gallon of cold water, applied with a rosed can to 100 square feet in spring".
Re: Killing comfrey Contact HDRA in England they will tell you what to use. Because it has such deep roots it is almost impossible to move I tend to grow mine in pots which helps when harvesting roots for creams ointments lotions etc.