DoraMoon is correct, when you delete a member their post will stay but the username associated with them will be n/a.
If you want to keep there username, changing this line in pop_delete.asp should do it.
Line 658 approx
'strSql = strSql & ", M_NAME = 'n/a'"
Add the ' to the start of that line.
I don't think that will have any side effects 