Use the group feature for your contacts.
Create a group for annoying salespeople and one group for all the other contacts.
Add the number to a contact you want to block and add this contact to annoying salespeople group. Make sure that your other contacts are added to the other group.
In the Settings and signals there is a setting for what numbers calls will be answered for. The default is all calls are taken. Change this by selecting only the others group.
I hope this solves your problem.