They pay on time every month which is good of course, but they borderline harass any contractor that comes to the house (landscaper, pool service, etc). 

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?

If the tenants are paying on time, not harassing you regarding updates, and generally quiet and good I would wait on at least one unit until they move raising the rent along the way as needed.  

@Josh Compton Your comments are a harass.

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."

Then the collections guys are harassing you and your credit is jacked up.

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

This will help matters when this comes before a judge.I would NOT, under any circumstances, harass, threaten, intimidate, insist on immediate vacancy, or hire two large men with shady pasts and expandable batons to hide in the bushes outside the apartment for an opportunity to put your problem tenants in the hospital.

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