The act of contacting them with a polite letter should only be the first line of attack. There are two concepts in play here: (1) Leverage and (2) Legitimacy. Sending a letter which identifies the misandry in their ad and requesting for the ad to be removed invokes the concept of Legitimacy. But if they don't share your feelings about the seriousness of combating misandry, they may not act. And so the next step is to pile on: send an e-mail that references the first e-mail, and again reminds them about the Legitimacy of combating misandry (this reinforces your first message to them). But in your second communication, add that if the ad is not taken down, you will hold a press conference about the company's intransigence (to make good on this threat, you'd have to contact a local MRA group in New Zealand, who would hold the actual press conference). Such a threat establishes your Leverage.
Further communication simply reiterates your previous communiques, reinforcing both the Leverage angle and the Legitimacy angle each time. If making good on a previous threat is nevertheless ignored, you should identify new ways of escalating your Leverage. Always make good on your prior threats to exert Leverage (and be able to make good on future threats), and remind them when you do so in future communications. An example of escalated Leverage would be to write to the members of the Board of Directors, and/or any other authorities who already have Leverage over their company (such as government authorities).
The cumulative effect is that pressure and momentum begin to build until they either cave in or (very publicly) suffer the consequences.