Hi
You have the possibility of selecting the partner type you want to see in the selection field by selecting the partner function as PE.
However one should provide the value for this normally. If you want this field to be greyed out, you have to only create a variant for each sales employee and ask them to use that variant alone, or you should copy the program SAPMV75A of VA05 and create a custom code to enable your requirement.
In SAP, I dont think you have the possibility of greying out a field with fixed value by default (except by the usage of variants)