Windows IT support

Tell them you don't have a computer

These are likely to be fake calls and NOT from Microsoft, Microsoft will NEVER call you unless you contact them for help first.

I suggest that you have a look at this old post to be more familiar with this fraud:

http://cyberdefend.spaces.live.com/blog/cns!2033D634A61595B3!300.entry

I would say that the "scam calls" and "ha! ha!" in Nan's message are more than enough of an indication that she does not for a second believe that the calls actually ARE from Microsoft.. But always nice of course when someone states the blooming obvious, Dave.
How to make them stop then.. That is more difficult. If Nan ever has a bit (or a lot) of time in her hands when a call arrives, playing along might be an idea. Just pretend to be the wold's most stupid person, speak slowly, ask them to repeat everything, ask inane questions (now how do I get my PC to turn on..), act confused.. The idea is to keep them on the line for as long as possible without actually giving them any info.
If you repeat this a couple of times they may stop calling..
Faster alternative is to hang up as soon as they say "Microsoft" with no explanations and not asking them not to call again (they won't listen anyway).

Say you are not computer literate but it is your husband who is the computer buff. Say they can contact him then give them the telephone number of the local police station. I experienced this. They insisted I turn on my computer and when I refused they got rather nasty about it - well they would wouldn't they? I just hung up.