This employee cannot be deleted because it is used in unbilled charges.

I have 2 employees who left the company years ago and I cannot make them inactive, as I get the above message. 

I do not use QB payroll functions, but have employees set up for our own payroll.

I have run reports to check for unbilled charges and time charges for all dates, but they return no results. 

I just want to remove these former employees from the main list and stop them appearing. Any ideas?

I'm in the UK.