I handle all the interaction, details, and harassing the flipper, they get their principal and return upon completion (only a few more weeks!) 

Cannot prove it, buy I feel I am being harassed now ...What should be my next action?

All the useful information have posted about financial reserves and so forth can lessen the risk of this too, but if somebody has nothing to lose, they can wreak havoc.There is the possibility of claiming discrimination, harassment, injuries from an unsafe condition and so forth.

(EX: dealing with bad tenants, finding good property managers, sexual harassment, etc).I appreciate your input, thank you in advance.

My question is: how much can I hound her for payment before it's considered harassment?

If she had not actually gone to the sheriff and called me right in front of him I would consider it, but I don't want to call her and then give her a chance to go back and say "see he called me again, he is harassing me."

@Josh Compton Your comments are a harass.

We tried polite reasoning with her, but she kept harassing us.  

What can I do to make her cooperate and quit harassing my wife and me?

Continuing to mail could be seen as harassment.