I knew I posted this question too quickly. I found the answer.
Setting "Call Pickup Directed Accept" to No of the DSS/BLF watched extension, prevents the host set from picking up that call.
In other words if 104 had a DSS/BLF of 105, and the COS of 105 has "Call Pickup Directed Accept" set to No, 104 won't be able to answer 105's line when 105 is ringing.