As hard as it might be to imagine DC has made canceling your license incredibly easy with the ability to do it online. Simply go to the following link:
Make sure you have a copy of your license scanned as you will need to upload it as part of the cancellation process.
In addition to your license, you will also need to make sure to close your Business Tax Account with the D.C. Office of Tax and Revenue (OTR). Your CPA or tax preparer can advise you specifically on how to do this. You may also speak with the Business Tax Division of the DC Tax Office by calling 202-727-4829.
MAKE SURE YOUR CANCELLATION REQUEST IS RECEIVED BY THE LICENSING DIVISION BEFORE THE EXPIRATION OF YOUR LICENSE. FAILURE TO DO SO CAN RESULT IN LATE PENALTIES.