If the timespan is no longer required but is still associated to an Assignments and the assignment has a status of not Live, then we need to remove the timespan from the assignments and then the system will allow the timespan to be removed.
Use the option list assignments to from the timesheet timespan list to view all the assignment which are suing this timespan.
Now go into each assignment - please note you will need to un-tick the Active items only if the assignment is currently not live.
Now change the Time template to one of the default settings and select the Update button.
You can now return to the timespan and select to Remove