In IMG - SD - BF - Partner Determination - Set up partner Determination - Set Up Partner Determination for Customer Master put the tick not modifiable, if it required you can make mandatory as well.
In end user side maintain the sales employee in customer master data - XD02 - sales area data - partner functions - Enter sales employee code in partner function then save the master data,
now when ever you create sales order automatically system will copy the sales employee code from Customer master data to Sales order and system will not allow user to change sales employee in sales order manually.