In User Maintenance you can create a Customization Group and assign it to the sales staff, say "sales" Then create a script and set at Pre-Validate on the Salesperson field:

if oSession.ASObject(oSession.Security).IsMember("sales") = 1 then
retval = oScript.SetError("Sorry you are not authorized to change the Salesperson No. Nice try!")
end if

You can also create custom panels for the sales staff in which those buttons are locked or hidden and not accessable.

Brilliant Thanks Louie. I will give this a go and let you know how it goes :) 

BigLouie, i think you meant create a role in Role Maintenance and assign the role to the users who should not be able to change the salesperson. 

The alternative is to use a common value in the customization group field in User Maintenance and then create a customization of the panel for that customization group and lock the salesperson field. I don't prefer this method as it can make upgrades and integration with extended solutions more of a hassle to deal with.

The script and role method is preferred because it is much more dynamic and doesn't rely on customizations set up on a per user or per customization group basis. You can even throw the script on the UI post load event and lock or disable the field so they never get as far as trying to change it.