you can use running totals, I would think, but that is DBlank's forte.
I use variables, which accomplish the same thing, just with more work.
What I would do is build a string of salespersons who are no longer with the company (either by name or id or some other field) like "|Dave|Steve|". For paranoia, I would put the delimiter at the beginning and end of the string (so NOT Dave|Steve).
since this is going to be for the entire report, we don't need a reset formula, but you need to set the 'bad' salepersons, so in the report header section I would do something like this:
shared stringvar badEmp := "|Dave|Steve|";
"" //hides the output
increment formula
shared numbervar aTotal;
shared stringvar badEmp; //your delimited string
if instr(badEmp, {table.employee}) = 0 then
aTotal := aTotal + {table.fieldToSummarize};
""//hide the running total
finally in the group you want to display the summary
shared numbervar aTotal;
aTotal
HTH