Not sure whether a vote for Winnie is a good strategy to keep the Greenies out of government. Vote National or ACT - Winnie is too unreliable and will go with whoever offers him the larger baubles of power ...
But than - I can't see any strategy which would make a vote for Winnie a good idea. If you want a Left government, than vote Labour. If you want a centre right government - vote National or ACT (if you want to support the centre).
What would a vote for Winnie give us other than an unpredictable government which will be the end of the rule of whatever party he is going with?