Though old Projects cannot be deleted, you can remove them from your dashboard to reduce clutter.
1. On the Developer Dashboard Develop tab, select the Project you wish to archive.
2. Select Settings > General from the left navigation bar, then select Archive my Project for ALL Unity services.
3. When prompted, enter the name of the Project to verify it is the correct Project to archive, then select Yes, archive it.
You can view your archived Projects and reactivate them by following these steps:
1. Go to the Developer Dashboard Projects homepage.
2. Click View Archived Projects on top right of the page.
3. Find the Project that you wish to reactivate and select Unarchive.