If they are threatening you it is because you are hitting a nerve. It's a sign that you are doing something right. Do you have their threats in writing? Recorded? It is my understanding (until I am corrected) that it is illegal to threaten legal action as a means of controlling someone. They can sue you, or not. They can't threaten to sue. It's a big no no.
Printed copy · Anthology Volume 2, page 964Printed copy from The Tender For Law Anthology, Volume 2, page 964 (printed 14 June 2014). Added by this site; not part of the Facebook export.