Is it possible to shrink a LUN?

No - its not possible. Since the Compellent is thin provisioned, you can have a partition table smaller than the LUN to "fix" a mistaken size. You can also do OS level partition resize/copy between LUNS to "shrink it". The issue of course is that the compellent can not see what the OS has done, except as provided by the Server Agent. Server Agent helps the thin provisioning stay efficient, but cant resize the volume down.