In Taskulu you can easily delete sheets (which are also called tabs) and lists. Although, you need to have a certain level of access to be able to do that.
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, or edit the title of the sheet by clicking on the pen icon.
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.