It's such a tempting proposition - and one that each community has treated differently. Back when Marc Fleury ruled the roost, JBoss forums were notorious for their low tolerance of newbies who didn't RTFM - or so I've been told. At an Open Source community managers' meetup at OSCON, perhaps the most cogent statement on the subject came from someone whose name I've unfortunately forgotten: the first time someone asks a question, be very responsive; the next time, take slightly longer; the third time, even longer, and so on. The thinking is that this approach encourages users to think for themselves.
In fact, it's a lot like parenting. Reward good behavior; punish bad. Sure, you *can* go negative (yelling, physically punishing child) and reap the short-term benefits of immediate obeisance, but you may end up laying the groundwork for more work in the future. It's better to make sure good behavior is properly rewarded, thus giving all members a clear indication of what behaviors the community encourages and those they will not tolerate. Sure, as community manager, you can ex-communicate whomever you please, but that doesn't exactly set the tone of trust that you want the rest of the community members to feel. As Matt notes, it might lead others to conclude...
what would those other 95 percent think if they saw a community member--even an obnoxious one--dumped from the forums? That doesn't sound like the sort of community I'd want to join, where your voice is only valid if you happen to be singing in tune with everyone else.
The community manager's role is to make sure that no one becomes a nuisance, so that does require some active participation, but not outright antagonism. The challenge is to nurture a culture of respect, where users will naturally shun those who are potentially abusive. The community manager must actively set the tone and demonstrate the rules of engagement for others. Eventually, those who earn the "annoying" mark will either shape up or ship out.