What do I do if I revoked the wage assignment properly and they sent another intent to take my wages to my employer?

How do I revoke a wage assignment. I bought a car was scammed the repairs I can no longer pay for. I will be filing bankruptcy, but am a struggling student so cannot do it yet. The company has never contacted me but sent a fax to the public fax machine at my job with an intent to take my wages. I sent them a certified letter stating that I revoke and this was not the proper process, so they sent a certified letter to my employer with another intent? Do I need to send another certified letter? What do I do now? My employer does not seem to understand the difference between assignment and garnishment. There is no court order only the wage assignment that I am trying to revoke but they do not seem to take no for an answer. This company scammed me.