In article .com,
"BobK207" wrote:

David-

The Dremel is a good way to go

OR

get a countersink that is large enough to consume the copper tube, chew
it back into the wall until you're satisfied

OR

a large rotary burr in a die grinder


OR

slit the tube in a couple places with a hack saw blade & then using a
hammer & small chisel, fold the tube wall back into itself.

How about this-

With a Sawzall, cut through the drywall and into the old copper pipe above
and below the stub and it will fall into the wall cavity (or you can pull
it out). You'll only have the stub's hole and two sawcut lines to patch.

--
charles