alstroemeria

My garden has a terrace which, when I moved in 5 years ago ,contained a section of alstroemeria.  It can be beautiful but not this one; this is a weak, sickly bendy plant with insipid yellow flowers, but it is spreading rapidly.  My dad and I spent a horrible day 4 years ago digging out tightly packed tubers and replacing the soil, to no avail and this spring I painted the leaves with roundup gel.  Neither of these has made any difference.  It sucks up new compost and chokes any other plants.  Any ideas?