Q: how do I lock my device if it is stolen and has already had the account changed using the sleep wake button and conneted to itunes

if my IPad was stolen can my account be erased and a fake one put in so it miht be used? even tho i have a pass code and ICloud? I use ICloud to locate it so far it has shown offline for 2 weeks.  I put it in lost mode and play a sound but it was gone for 24 hrs before I did this. I also have ITunes on it.