How Can I Stop Getting Stupid Phone Calls Saying I Have A Virus On My Computer !?

Quick answer is, you can’t.
Long answer is -
1) Make a note of the exact time & then inform BT you are getting abusive calls, give them the time of the call and ask BT to block the source (BT will, of course, try to SELL you CALLER ID and ‘Call Blocking’ for a FEE, however they are REQUIRED BY LAW to block ABUSIVE calls for free .. (not all Call Center staff know this ..))
2) Same as above, but now tell BT the calls haven’t stopped so you would like them (BT) to assign you a new EX-DIRECTORY phone number .. you can be charged a ‘connection fee’ for this (and if your Internet comes down the phone you will have to pay to reconnect that too, or switch to cable)
3) Dump your ‘land line’.
Make sure all your fiends + your Bank etc have your mobile phone number, then unplug all your phones … switch your Internet to cable as soon as you can then tell BT you no longer need their services and you will no longer pay for line rental.
WARNING – already fake and ‘phishing’ TEXT messages have started to appear on MOBILES .. so STOP giving out your main (monthly contract) mobile number now .. use that for friends & family only.
Get a few PAYG SIMM’s from the 99p stores (or send off to one of the ‘air time’ suppliers who give them away for free) & put one in an old phone and give that number to companies & web site ’sign up’ who demand a phone number (& especially for those that have no valid reason for it other than spamming you).
You don’t have to put any money on the PAYG SIMM since it will only ever be used to receive incoming calls (eg to arrange a delivery). When you start getting too many fake Text messages, throw that SIMM in the bin & put in a new one.