When you make the staffer inactive HelpSpot it tries to assign and assigned requests back to the inbox. However, since you have random assignment enabled some requests are being assigned back to the person before they are being made inactive. This is a bug and we’ll have this fixed in the next release. In the meantime you could create a filter that shows all open requests, add the assigned to column to be displayed and reassign any that belong to them.
Admin, you’ll need to go into the database and set the fDeleted column to 1 in the row that contains the old user in the HS_Person table. You can also use the above technique to find any requests assigned to that user and change them to someone else.
Note that old requests they inactive user worked on will still be assigned to them. This is so that HelpSpot can keep an accurate history. You should only worry about changing any which are still open.