Ok, let's get one thing clear. I don't dislike these people personally. It's a job. Their job is to ring people up, unsolicited and try to sell them something - anything, to make some company some money. OK, so it's capitalism and that's fine, but if someone is on my phone and I never asked them to be on the end of the phone, then I think I have a right to decline the call, cut it short and get back to my life. These people tend to be pushy too. Sometimes, really pushy, and that's when I think you're entitled to take a more direct approach to the situation and instead of someone wasting your time, you should see it as an opportunity to either waste their time, or at the very least get some enjoyment out of it. Remember, they already know your phone number so you've got little to lose, and the pleb on the other end gets so much rejection he wont bear a grudge. Ultimately, the more outrageous what you do, the more fun you'll both have. After some of these, the person who calls you will have something to tell his/her friends when he/she gets off shift.
The most common suggestion is, if you dont mind having your phone engaged for a while, tell the person you're interested but you just need to switch the kettle/dryer etc. off, and either put them on hold, or just set the receiver on a table and go about your normal day. The best thing about leaving the phone on the table is that they can hear you listening to TV, calling friends on your mobile and just *living*. Eventually they'll hang up - they have to, otherwise they'll miss their quota. Some people think that keeping them on the line means they're wasting their money, but most aren't as they're on flat rate leased lines - remember, the telco companies love these companies - to them, they're good customers.
If someone is selling you a product, and if you have some free time, indulge this person by questioning every feature of the product. If it's a satellite TV system, or any kind of tech gadget, ask about stories of it being links to cancer due to gamma waves. Ask if it works with black and white TVs. If it does, ask them where you can still buy a black and white TV. In widescreen.
I did this one because the the guy calling on me kept asking for a call back so I decided to have some fun. I asked him if he was on the phone all day doing this (in a sympathetic voice), and he gave me the company line which was to say he was just doing a turn. I asked him if the headset hurt his ears. If it did I knew a brand which could help him, and then you need to fly into TV commerical voice over mode: "Do headsets hurt your ears? If they do, try new Eazee Earz (TM) with it's patented ear cushion ergonomically supporting your ear, you can get through hundreds more calls in a day. If you buy today we can give you a thirty percent discount depending on availability. Remember, if you want an Eazee job, get Eazy Earz!" Don't forget to fully pronounce the (TM) section as 'tee-emm'.
Often, some junior person has to ring up 'potential clients' (i.e. cold calling) for his boss or a senior sales rep. This is common in the 'financial services' industry. I know these people want to get on, but hey, that's life. If they're asking for an appointment, ask who it's with, for example Jon Smithington Smythe or something. Tell the caller that you think you saw him once in your local gay club (sorry for the slur there guys, but I hate to say it, most of these people who call be are pretty homophobic). If it was too wild a statement, say you might be mistaken, but you've got his card somewhere. Any similar thing will work too - tell the caller you remember meeting him a few months ago when he was drunk as hell with a stripper and he promised he wouldn't call you.
Tell the person you've always wanted to just call someone up, and ask what kind of qualifications they need to be able to do that job. Ask for his company's HR department phone number or address so you can apply. You see, they're not supposed to talk too much about the company, and asking for a physical address will probably start panic. If they give you an e-mail address, make sure you post to a lot of really dodgy usenet groups and forums with it as soon as possible.
I know most of the things on this page are probably examples of broken logic, but what I mean here is that you need to talk to the person about things that have nothing to do with what that person says. If they ask you about investment accounts, ask them how boiled eggs can make life better for people in Chile, and why are you calling up to ask that. Tell them you don't speak [whatever language you're speaking]. This will usually get a pause, during which time you should call to an imaginary 3rd person 'Hey, do you speak [spoken language]?'.
For a snappy exit, try one of these 'situations'. Some of these require you hanging up, which when all said and done is a bit rude, so save them for emergencies and repeat offenders.
"Sorry, I'm beng held hostage for ransom now. Can you call back later after my husband/wife/family has paid the ransom? Thanks" [click]
[plane sound effects] "Oh my God, we're going to crash!!" [click]
"OOh, You've got a nice voice, what are you wearing?"
"Why are you calling me? Did they tell you to call me? Are you watching me??? STOP IT!!!" [click]
[fake sex noises] "Sorry, I .. ow, that hurt!...can't talk now. Can you call back when he/she's finished?" [click]