You would control this in Accounts&Privileges.
In the privilege set for your sales reps, on the left side, you have record privilege options.
Choose Custom Privileges and set the calculation to:
Table::sales_reps = Get(AccountName)
Note (obviously) that the value put into the field has to match their login account name.
You may also find it more convenient to put a "find by account name" step in your startup script. This lockout process does not stop folks from seeing the record itself, but stops them from seeing the data on the record. So they will come across a bunch of 'blank looking' records or fields with "<no access>" if you don't start them off in a found set by their account name.
I also opt to put a "find mine" button on the layout that does a find in the sales_reps field for "Get(AccountName)" to help.
I am using FMP 10. How can you filter out the results that say <no access>? Can you be more specific on how to filter out the records that say <no access>.