LockGuides XT is a free XTension courtesy of Markzware.
Per a user's request for the ability to lock guides, since we all experience accidentally grabbing and moving them, LockGuides XT allows you to lock an individual guide by double-clicking on it, in which case a dialog will appear asking you if you want to lock the guide. Once locked, a guide cannot be moved.
LockGuides XT also allows you to lock or unlock ALL guides by selecting "LockGuides XT..." from the Utilities menu. Note that locking or unlocking all guides does NOT affect any new guides you create in the future, so it is perhaps best to first lay down all the guides you want before you set the lock status.
Note also that LockGuides XT will need to be active (in your QuarkXPress folder upon launching) in order to unlock the guides of a document that LockGuides XT had previously locked.