They never came back. Tell them you are a Catholic priest who would like to infiltrate their organization and bring them in subjection to the pope. Pull an AK-47 out from behind the door, point it up in the air, and ask them if they have ever seen such a fine military weapon. Repeat this until they leave. Agreed, we are not prepared for this one, but it has to be carried off perfectly.
Automatic weapons are undeniably the best deterrent to Witnesses.
For extra effect, fire a few rounds into the air or towards their car. Pretend you hear a voice inside your head telling you to kill the witness. Guaranteed to stop future visits for several years. The young couple came to my door. I was wearing my robe, and had just awakened.