To stop those calls from companies who call you up to try to sell you something, you can try register your phone number with the Telephone Preference Service (TPS). Registration is free, so its worth a shot. By law all companies must screen their numbers against this opt out list.
As they describe themselves
"The Telephone Preference Service (TPS) is a central opt out register whereby individuals can register their wish not to receive unsolicited sales and marketing telephone calls. It is a legal requirement that companies do not make such calls to numbers registered on the TPS."
Whether or not this stops companies from wasting my time, remains to be seen. However if any company does ring me now, I'm going after the £50,000 bounty -
To help make it a little easier, here is a link that takes you straight to the opt-out process.