They can never just leave the church alone ever. I think it's fair to say, that we need to pick our battles carefully. Don't go to anti mormon websites and try to convince them, it just won't work. In fact, I don't think interaction with anti-mormons is very productive, since the majority of them won't want to have anything to do with the church. Why try to reel them in? Hearts can become much too hard. There is the off chance that they come to to christ. I met a guy who was a member on my mission who used to be a hardcore anti. Although he was never a member and went apostate, he was just a hostile protestant. Its definitely an exception, not a rule. I think the best thing we can do rather than contend with Anti Mormons, is to focus our efforts on being disciples of Christ, and sharing the gospel with those who are prepared to hear it. Sharing with those who are humble and willing to repent, that they can come to Christ, is the goal of the church. Our time is much more valuable, spent in this endeavor.