If you meant to protect your library from being modified, then add a password on them
If you meant to protect your library from being manipulated, copy the guid number of every object and protect it with a password.
(Guid, a special serial number that identicate when was created, with which archicad's version and when was created as an object ... Besides, this number changes with every time the object is being saved)
If you meant to protect your library from being copied or taken, well I guess that it will be a little bit difficult and most of the time unnecessary
A good friend of mine have once told me that I´m so brute that I´m capable of creating a GDL script capable of creating GDLs.
You can lock object with password and request the AC key number in the script (eg master script).
If You give/sell the object to a AC user all You need to put the key number of the one that will be using it.
If done right, user of no other key will be able to use the object.