Have you looked at Responsibility Centres, they may not do exactly what you want but would be a good place to start. Take a look at how they work, and then get back to us here if you need more help.
To get started, create a Responsibility Gorup called SALES, and then assign it to the users in that group, and then on a few sales orders also assign this responsibility centre. You can also create a new Sales Order whilsts logged in as a sales person, and the order should automatically be assigned to that role.
If the user does not require lots of screens or reports.
You may want to add some coding to tell the system to display the report with the salesperson code in the table filter.
Even if the user runs the report, your filter will supercede their userend filters.
But go with the granule David stated as well.
If you go the route with the coding. Make sure Navision security give the saleperson only rights to that report/form you modified.