You don't. Non-professionals will become professionals with time and experience, but there will always be more non-professionals to take their place. The only reason some communities have to deal with more non-professionals than others, is that those communities are perceived to have a lower barrier to entry--so, when a non-professional tries to select a community to join/language to learn/tool to use/etc., they end up with the "easier" one, and thus the "easier" community builds a reputation of non-professionalism. The only real measure one can take to avoid this is to make the community unattractive to non-professionals: in est, to raise the barrier to entry.

I don't think barrier to entry is the only factor, if it's a factor at all. The Linux desktop has a relatively high barrier of entry, and yet a large part of its community has devolved into an elitist, cognitive dissonant crowd that's defending their usability problems by framing them as a good things.