I suggest configuring iptables to block Gaim's ports at regular intervals. Chmodding would do the trick, but it can be circumvented quite easily. If you want to keep your small children (or your wife/husband
out of Gaim for a while, chmodding could be enough, however.
I think you should also install anacron, because otherwise, if the system is down during the time when the chmodding/iptabling should take place, it doesn't. It's only a matter of time when users realize this.
Anacron handles cron jobs that have been skipped due to downtime.