Repopulate more predators like dingos who hunt them to let it naturally take course.
Inject them with animal birth control. It lasts up to 2 years and works!
Your question is more for vegans though. A vegan life style is to prevent animal cruelty in the human world. A vegetarian just eats fruits, vegetables, and sometimes will even consume dairy products or fish. Their reasoning is to transition to a vegan life choice, they don't like meat, or for health reasons.
That all just seems like a very expensive and redundant way to achieve exactly the same thing, i.e. the slow death of kangaroos. I get its an option, but I don't see how it's a better or even more ethical option for roos to die by dingo rather than gunshot.
Then your question can't be answered. You were given solutions. ---