When I feel particularly annoyed at them I like to lead them on sounding interested then asking them to hold on a minute and just leave the phone off the hook till they give up I feel I am helping the next person keeping them busy with my call. I had one on the line for over five minutes, I could hear him calling 'sir are you there?'
The creative approach is the best approach I think.
The telemarketer is just having a shitty job, and he or she gets the numbers handed by supervisors and higher up.
When in the mood, I'm getting them to talk about their job, somehow they always seem to be looking for another job.
Or I ask the most silly details, and after getting lots of answers, still tell I am not interested. One day I am going to have a massive text to overwhelm them, before they can try doing that to me. Answering in another language, or telling I will go and find my mother is still in my planning too.
I can be in more rude moods too, of course. Bad luck for them.
Once, long long ago, I had a shitty job like that.