I think this is why the OP mentioned buy less stuff and travel less, these two directly reduce the demand for environmentally harmful goods and services, reducing the ecological impact of the companies which issue the shares that make the billionaires in question billionaires.

It's kinda disappointing to see a post about good actionable advice to do the best you can to reduce climate change and the first reply on Lemmy is non actionable (and more controversially; to my mind irrelevant) advice to assassinate billionaires.