Prior to Clarity 16.0, if a Resource was leaving the company, I would remove all his investment ETCs from the Resource Allocations portlet by selecting the investment and clicking on the Remove button.
If the investment would have Actuals, I would get an error, but the ETC of the Resource would be zeroed out.
PRJ-07580: One or more staff members cannot be removed. This may be due to security, committed actuals or booked requisition.
In Clarity 16.0 the ETC does not get zeroed out
A design change was introduced when there are Actuals on an investment to prevent Dependencies / Conversations from getting lost.
An alternative would be to build a portlet on the Assignment object.
The ETCs for a given Resource task assignments can be removed in one go by editing the ETC cell on the grid list.
Please see below what the portlet would look like: