Best not to say anything threatening. 

My worst most filthiest disgusting tenants who caused me the most aggravation & called me screaming and threatening started out complaining before they moved in with small stuff that didn’t need to get fixed. 

The difference is that you didn't say there would be a teensy increase, and the tenant threatened to move out if you did - and then you tuck your tail and say, "Oh, no!

I bought my first rental property in May in Huntsville, Al, and now the insurance company (State Farm) is threatening to cancel our policy because the property has been vacant for over 30 days.  

The post office will then put a sticker on it with their new address, and return the envelope to you.http://pe.usps.com/qsg_archive/pdf/qsg_archive_201...What my daughter did over her PMC disaster, is threaten to sue them and that worked for her.  

But, it's my opinion, that legally you aren't liable for dust or damage to their stuff.If they threaten to sue you, what I'd do is say, "Well, I don't think it's my responsibility, but it does make sense to let a judge decide, so you go ahead and sue if you want to, and I'll abide by the judge's ruling."  

I have told a few threatening lawyers to more or less shove it where the sun don't shine, knowing full well they will lose in court.

I would threaten the eviction.

Don't let her threaten you, you are the one who can enforced any rules at your property. 

Saying C is inherently irresponsible, and threatens the viability of a real estate investment because he has no cash - is not your best approach either.