Yes, decommissioned accounts can be excluded from Account policies.

Accounts that have been decommissioned will show up as "No Current Billing" in the Policy report.  The recommended best practice is to rename deleted accounts with a prefix such as “deleted-<account name>”.  This makes it easy to identify and ignore Critical status on deleted accounts. 

After a policy has run for the first time, you are able to select accounts to exclude from future evaluations as shown in the image below by clicking on the “Exclude” button. 

To do this, go to the Policies main page and click on the eye icon preceding the policy of interest. Then click on the severity level to expand.