Are you following the below steps?
1. Navigate to System Definition > Tables.
2. Open the table to delete.
3. [Recommended] Click Delete All Records.
Deleting all records before deleting the table ensures that the business logic is properly executed (for example, reference cascade rules or other delete business rules). If you do not delete all records from the table first, then you must manually fix any other records or tables that are impacted by the table deletion.
4. Click Delete.
5. In the confirmation dialog box, enter delete and click OK.
The table and all items that reference the table are deleted, including:
- Choice list items
- Forms, form sections, lists, and related lists
- Reports and gauges
- Reference fields that reference the table
- Access controls
