Over time most members develop an understanding of community norms and what constitutes acceptable behavior. These norms reduce the need for a community to police itself or reduces the number of actions taken against unacceptable behavior. However, in the beginning most communities don't have this informal management.
It may be necessary establish a core of members who define good practice and mentor new members. These individuals serve as moderators for group activities, guides, or even teachers of certain practices to develop the group overtime. If possible seek volunteers who strongly support the community objectives, but often paid staff will be necessary.
Special membership requirements or dues reduces disruptions among members, also clearly defined community objectives help greatly. Members will come to respect the interactions between members and work in the benefit of the whole because subconsciously they don't want to impose on others.
Communities are improved by multiple channels of communications. Being "on-line" is just one way to connect with members (or between members)-- but to say "on-line community" is as ridiculous as saying "telephone community".
For a constructive community to prosper members must be able to experience it in what ever way is convenient to them. This may include print journals, on-line collaboration, telephone conferences, or even networking events. The easier you make it for members to connect, the more value they place on their relationship with the group.
If you focus on a single medium your community is likely to stagnate and weaken over time.