If this person is making false written statements against you, you can sue for defamation. If they are also speaking these things you can sue for slander.
You should send them a cease and desist letter as soon as possible. You might also be able to get a cease and desist order, but this is hard to do, and can be costly.
The best option is to do the cease and desist, on an attorney's letterhead. We find that usually slows down the person who is making the false statements, and helps to build your defamation case by putting them on notice of the untruthfulness of their statements.
Get a good lawyer of your own ASAP.
PLEASE NOTE: The responses made here are for information or education use only, and are NOT legal advice and do not form an attorney-client relationship! Only licensed attorneys you hire in your state can provide legal advice.