The problem is that the encryption is easily reversed if you're determined enough, it might stop a casual person but anyone with any tenacity or google will be able to bybass that quickly once they have a script.

This is one thread on MSDN about it though it appears that you need to have Admin rights.

http://social.msdn.microsoft.com/Forums/en/transactsql/thread/e7056ca8-94cd-4d36-a676-04c64bf96330

Lowel is correct you're better using Permissions to prevent this from happening and if it does happen using an Database trigger and Audit to trace changes so you know who to point the finger at and prove that someone has done something they shouldnt.