It wouldn't be in her best interest to post anything negative about your company. If she has done this before, there will be several replies to her post from other companies she has done this to before.

Hopefully she is smart enough to realize that if she decides to take action on her threats, she may the one hurt more by her own negative post.

If someone on here post about a non-paying company, several people reply with their own experiences with that particular company, whether good or bad. If someone doesn't believe it to be true, they will say "we have never had any payment issues". On the other hand if someone had the same non-payment issues, they will voice their opinions.

I would suggest that if she post anything, you should reply with you side, in a very profession manor and other people that have had issues with her in the past will follow suit.