For the non payment of rent I just told them that I would be sending a legal notice for non-payment of rent and she replied right away all angry at us for threatening and that they will pay tonight.

Even with the note being paid, the banks will still send threatening letters.

I myself have received threatening voicemail from individuals with thick foreign accents telling me I owe thousands to IRS.  

The day after I wrote the last post the owner called me hysterically telling me her son was threatening to kill her and burn down the house.

You cannot touch somebody's credit without first taking them to court.I had a commercial landlord threaten me with that once.

So threaten eviction, if they dont pay, then move to evict.

Another is to write and state clearly that they are in breach of contract and are the authors of their own misfortune, and that if there are any more threatening communications we will be forced to take action.

I would even go so far as to threaten legal action if she doesn't by some deadline (24-48 hours).

I finally ended up having to threaten pulling the buyout option because he kept delaying on signing the agreement.

The reality is that most listing agents will not do itWe can all yell, scream and threaten but the system is the system.