I'm guessing you are speaking about blocking of incoming call from particular numb. Right ?
ATA is rather simple end-device. It have very limited set of features. Complex call processing belongs to PBX.
Cisco staff is not monitoring this community as far as I know. Feature request like this one shall be discussed with Cisco SMB support. Paid support contract may be required for it, and even with it, your chances to push new feature into firmware are rather low. So my best recommendation - you should implement blocking on PBX.