If the question is what a service provider (i.e. webhost) can do to reduce the likelihood that a malicious link is posted on your site, there are a few available methods. I recall that Websense offers a service, Threatseeker cloud, a web service where service providers can use to verify whether a link (shortened or not) belongs to any number of pre-defined categories (i.e. adult, malicious, etc). I'd guess there are other vendors and ventures aimed at providing this type of service to service providers.
Aside from checking each posted link before its published, you can always look at other technical controls mentioned in the other answers or perhaps a simple administrative/social control to ensure only registered users can post. You can add on the requirement for moderation by a moderator or add on the social component to require posting of links only be available to registered users after some measure (i.e. positive votes, reputation, etc).
Lastly, you could always mask links so the user has to copy/paste it into their browser. But this approach moves the burden back on the user.