In Taskulu you can easily delete sheets (tabs) and lists. Although, you need to have the right access to be able to do this.
For deleting a sheet, click on the gear icon and select "Manage Sheets". Sometimes sheets are hidden and you need to click on the chevron icon to see all the sheets and the gear icon.
In "Sheet Manager" window, hover on the sheet you want to edit. You can delete a sheet by clicking on the red bin icon, and edit the title of the sheet by clicking on the pen icon.
There is no direct deleting for lists
If you want to delete a list ,first you have to archive it and then from "archived tasks" page you can delete the archived list.