You should probably talk to your client before doing anything, and find out what they want to do about it.  Here's what your client needs to know:

1.  Any $ paid to employees, including unrecovered advances, should be T4'd.  The downside is that this means CPP and EI deductions at a minimum.  The upside is that it effectively converts the advance from a receivable to an expense, which can be deducted when calculating the business income tax.  If your client wants to go this route:

1a. Sage doesn't allow you to go back and amend T4's from years ago, and as you likely don't want to do this anyway. 

1b. So, the workaround is to re-activate these long-gone employees and pay them each a bonus large enough to cover the required CPP and EI (I don't deduct any tax on this type of bonus...).  The idea is to deduct the old advance and make their net pay zero.

2. If your client doesn't want to issue T4's for these amounts, then you can remove the unrecovered advances like this:

2a. Re-activate the long-gone employees.

2b. Do negative paycheques for each of them... just recover their advances.  The result will be that the advances are removed from the advances receivable account (credit) and added back to the bank account (debit).

2c.  Do a general journal entry to credit the bank account and debit some account or other.  Since the amounts haven't been T4'd, your client will be on slightly iffy ground claiming them as payroll expenses.  It is safer to put the debits to shareholder's loan or owner's draws... effectively converting the old 'loan' amounts to 'personal gifts from the owners... not payroll at all."