I saw a good one on a US site for this sort of thing - it said that everyone did this it would feck em up for good
When they phone and start their spiel then you interrupt with a comment such as "Sounds like a very interesting offer -i want to hear more - let me just turn off the spuds" - then leave the phone on the side and piss off - if everyone did this they would wait 5 mins b4 ending the call, thus reducing the amount of calls that can be made and the cost effectiveness of telesales
with the junk mail - put their schtick in the envelope provided and send it back to them - if everyone who they sent it to did this it would double the cost of thieir mailshot and also increase the staff costs for return post sorting
these sort of things only work if nuff peeps do it though