I very much doubt the issue is due to the Guest account being hacked. Anyone not registered or logged in runs under the Guest Account so everyone has access to this account.
Guests or in other words the Guests Account then has the same permissions placed on it any other account would, but with a few extra restrictions added such as not being able to send PM's and having to use CAPTCHA to post.
Guests can only post if the permission system allows Guest posting. Check both Group and Member permissions that some one has not mistakenly allowed Guests to post.
You may also be mistaken that a Guest has posted. If you delete a member from the admin area, so that topics do not get 'disjointed' with missing posts and posts created under the account would be shown as being posted by a 'Guests' once the member is deleted.
If you are not running the latest release you should also upgrade your forum as allot of work was done to protect the forum from cross form forgery requests where hidden links, images, forms, etc, can be used to trick a registered user in to submitting content of changing admin settings without them knowing.
Edited by WebWiz-Bruce - 25 November 2010 at 3:51pm