If it truly is only the SIM that is blocked, you can simply get a new one. Most providers sell them for $10 or less.
Now if it is the Iphone ITSELF that is blocked, there's nothing you can do. And yes, it IS possible to block the phone from being used (It is just not available as a consumer option).
If it is the phone itself being blocked, there is an unfortunate possibility that it could have been stolen prior to you getting it. Or it could have been incorrectly marked as stolen from a dealer (if for example, the sales rep who sold you the phone misplaced the paperwork and the phone was no longer in the store inventory).
Providers in North America now have access to IMEI Blacklisting, just like Europe.
IMEI blacklisting is a means of registering the IMEI of a handset stolen from a dealer on an international blacklist which is shared among mobile companies. This essentially prevents the phone from being used on any GSM capable network around the world, rendering it useless as a phone.