smithsjt has it right in that if you have the junction table, you have a many to many.

For a one to many customer to salesperson, you really have CustomerID in the SalesPerson table. Not the other way around. If you have SalesPersonID in the Customer table, you are saying one salesperson per customer.

I would leave it the way you have it. You can add logic or a trigger to ensure one customer per salesperson, but this is one of those business rules you have to expect to change. Having a salesperson only work with one customer is rare. And it often changes if you get a really small customer and you make a salesperson then have two customers.