We need professional assistance. Can we sue for this action?

We are a community non profit who are being seriously harrassed by someone's ( a well known figure ) who is deceased that we were going to help with a program that never started. We always had our EIN 501c3 status and now years after his death she is posting, blogging, and announcing on radio shows that we are stealing and defrauding. She also posted our personal names as well as called and left nasty messages on our personal phones. What can we do. This year our org was unable to obtain any sponsorships and we suspect this was due to their posting and publicizing this false information about us. info@suitlandfestcdc.org