Cut it off a foot or so above the ground then pull the upper part of the vines away from the wall, hopefully it will come away in large sheets (never as easy as it sounds) sometimes it pays to leave for a few weeks once you've cut it as it tends to lose it's grip a little.
Now you need to deal with the lower bit at the bottom of the wall as it's likely to resprout. There are a few chemicals on the market that you can use to treat the cut ends to kill it down to the root- Growing success ivy killer, SBK Brushwood Killer. root-out or deeproot. and follow the instructions on the individual product. Good Luck Tbird+