Our Rental fleet was losing money for years on the service side. In the past, a tech had carte blanche to fix any rental. No customer authorization needed, so hours were poured on the bills to make up for lost time if a tech was short on their 40 hr week. Now, other than basic Scheduled Maintenance, a tech must call service manager or account manager for approval on any repairs over $100.
The rental manager knows what is warranty and what's not covered. They often trim a work order's hours if deemed excessive.
It was common to see a "paper Clip" installed with 2.5 hrs labor, or drive unit gear oil changes with only one litre of oil.
Your avoidable repairs have to be well defined, sort out what is normal wear and tear from abuse.