Let's say their extension is 123, create an extension for them in extensions.conf called blocked:
[blocked]
exten => _1NXXNXXXXXX,1,Hangup
exten => _NXXNXXXXXX,1,Hangup
exten => _NXXXXXX,1,Hangup
exten => 123,1,Hangup
Then in sip.conf change their context to:
context = blocked
perl -e 'print $i=pack(c5,(40*2),sqrt(7600),(unpack(c,Q)-3+1+3+3-7),oct(104),10,oct(101));'