Deleting a Custom Object Definition
Instances of custom objects become rows in the T_PROJECT database table. If your database has such instances in existence, you must delete those rows before deleting the object definition. Locate the Y_OBJECT_DEFINITION table row that corresponds to the object definition that you wish to delete, and note the value of column APPLICATION_ID in that row. Then run a manual SQL query that deletes T_PROJECT rows where column APPLICATION_ID contains that value.
In addition, if you are running Data Warehouse, ensure that the Data Warehouse synchronization job runs after you have deleted the T_PROJECT rows, and before you actually delete the object definition itself. Data Warehouse tracks any deletions that are done against T_PROJECT and it cannot properly process those deletions if the matching object definition is already gone at the time that the synchronization job runs. After synchronization is complete, you may delete the object definition.