preventing certain phone number

[#1]

Is it possible to put a stop to a nominated phone number, if so, how is it done?

[#2]

If it is a nuisance number / abusive caller, you can report it to the police but the chances of anything being done are remote.
A better alternative is to use a call screening or blocking phone from BT - these phones let you block anonymous calls, international call and specific numbers.
http://www.productsandservices.bt.com/products/landline/phones

[#3]

The CPR call blocker can optionally block numbers as follows
1) International calls
2) Withheld numbers
3) Groups of numbers
4) up to 100 individual numbers
you can allow or disallow all or any, costs about £40 by mail order, my junk calls have stopped dead.
but you do need to consider that some people legitimately withhold their number.
Finally it is a bit sensitive to the quality of your broadband filter, so best to replace that at the same time.