In all likelihood, the mailboxes will be common property. I’d send the “disruptor” a note telling her the lock should be removed within 24 hours or it will be removed by whatever means necessary and she will be charged for any damage to the common property as a result.
I assume your block has a by-law forbidding Airbnb rentals. If not, time to get one. If so, time to start pinging the disruptor with Notices to Comply and breaches of Section 258, which come with a maximum fine of $550 per offence.
Oh, and before you do anything else, take some screen shots of her Airbnb page describing herself as “the disruptor” and “rule-breaker”. They will go down really well at NCAT.