Resignation aside, I always thought trying to share the historical problems (polygamy, BoM anachronisms, Book of Abraham, stone in the hat, etc) with Mormons/missionaries when they knock on your door is the most effective way to make Mormons stay away.
If there's ONE thing Mormons hate the most is reading "anti-Mormon" literature. Its like kryptonite for Mormons. They are TERRIFIED of the truth. They do NOT want to be around anyone that is going to share these inconvenient truths with them. When they perceive you as an apostate, with an "agenda" to crack their fragile little testimonies, they run for the hills.
What better way to put you on the "no-contact list."
I served my mission in Utah and when we came across a person who had anti-Mormon literature, we pretty much got out of there as fast as possible. When there were inactive/former members or non-Mormons who tried to share the truth with Mormons, they were ALWAYS marked as a person for missionaries and ward members to AVOID. There was usually in BIG BOLD RED marking on their address "ANTI." Inactive members who simply acted angry at the visits no one seemed afraid of. We always thought we could eventually love bomb them into being active again. Their hearts just need to be softened.
So I say either try and hand them some "anti" literature and or keep blabbing to them all the problems. They won't stick around for long!
My theory is that you say to the offending Mormon: "Look, you know I'm not interested in being LDS any longer. I've thoroughly researched the subject and am convinced it's a fraud. However, if you'd like to know more about WHY it's a fraud and how it's damaging to believe in, please come back. I'll prepare some information for you and have it waiting in case you decide to return and I'll assume if you do come back, it's because you want me to help free you from Mormonism. In the meantime, I'll pray for you."
Similar strategy here. I just said, "You don't want to talk with me." Then added a comment about the specific subtopic I was working with at the time. Briefer, but did the job.
When I first had my "crisis of faith," there appeared a concerted effort, led by the high priests, to rein me back in. As each one of these fellas gained access to me by various fake friendship means, I would give them clear and reasoned explanations of why I no longer believed. I used actual church sources, not peripheral "anti" sources to make my case. They ran like rabbits. It was over in a fortnight.
You don't have to use any anti-Mormon material. If you use truth about real history they will think it's ant-Mormon, anyhow. A lot of real Mormon truth is way out there in left field and doesn't sound like anyone would dare believe it.
I work at Hill AFB in Utah. A young airman was working with me for awhile. He was from PA and was getting a lot of "fellowshipping" from Mormon coworkers to go see the Ogden Temple. He asked me what to say to shut them up. I said to just let them talk and then ask them what their Temple name. That will shut them right up. He tried that and said that it worked. He said their faces went pale whenever he asked them that.