You can't bar specific numbers like that, the choices are;
Call Barring Service, this is chargeable though and requires putting in codes and pin numbers, as you mentioned your nan has dementi so I wouldn't recommend this.
I am pretty sure BT offer an Administrator Controlled Barring Service, i.e. they restrict her line so the only calls that can be made are to 0800, 0808 and 999.
You only need the pin number to remove or change the call barring options. Once you have set the correct options, you can just leave it alone.
There is some information here http://www2.bt.com/static/i/microsite/help_and_tips/faults/troubleshooting/barring.html
You should be able to add call barring into the line for £3 a month
An advisor on the phone should be able to assist to get what type of calls you want. It might also be useful to block 09 numbers aswell as the directory services