assuming the valve isn't permanently stuck open (and if it is, a bit of percussive maintenance with Special Tool No. 1 may repair it) all you need to do is block off the vacuum pipes at the modulator unit (usually fixed to drivers side wing). No vacuum = no valve operation
Last one I did I pulled off the pipes (feed pipe to vacuum pump, and the pipe to the valve) pushed a 3/16" blind pop rivet into each connector, small cable tie around the outside to keep it a tight fit and cable tied the pipes out of the way. Total cost about £1.00