We wire bathroom fans with the lights to prevent tenants from using the showers without having the fans running. It means they are noisy, and of course, we cannot ensure they use the fans if they don't turn on the lights to shower (during the day).
We live in Brisbane, so mould is not such a big issue for us up here, but we did have one mould situation a couple of years ago where one tenant was fighting the problem on her own. She was a naturopath and knew what to use to fight the mould, but it just spread along the ceilings through the house.
We had a roofer check the tiled roof. He said it was "delaminating" and would need complete replacement for the cost of about $24K (from memory). We had a second roofer look at it and say that was rubbish, that it was fine. I think it was simply we had a lot of rain. We've had that house over 20 years with no previous issues.
The tenant had been using oil of cloves and our painter used something (bleach product that he used and added something to the paint to help stop mould) and we have had no further problems since then (18 months ago).
So once it starts, it isn't always the tenant that "allows" it to spread and not always the tenant's fault. I've always heard Shannon Lush say that bleach doesn't kill the spores, but just bleaches them, but the bleach product worked where the natural product (oil of cloves) didn't.
Fingers crossed it stays gone.