This is the biggest catch 22 question out there and it can be both in good fun and can lead up to torture. As mentioned before by Susiestar, TerryJ, and lots of others, the problem is the laughing. When someone is being tickled they are laughing and while begging to stop, how can they take it seriously?
Also the catch 22 part comes with this. Suppose someone asks you if they are ticklish, if you say yes, that more or less gives you the green light to go ahead. If they say no, they'll do it to you anyways and I don't care what anyone says, you have a ticklish spot somewhere (like the feet or sides/ribs).
Now, this is related to my other post about the kids playing a game and how I have seem my kids play like that, but don't want them tied, but when I was a child, I had an older brother do this to me. It all started when I was trying to watch TV when he came in and wanted to and asked if I was ticklish because I would get it. The next thing I know when I get up, he sneaks up behind me when I come back and pins me down. He said he is going to count my ribs, and starts stroking his fingers trying to "grab" my ribs and started counting, 1, 2, oh sorry I lost count, that was torture.
A few weeks later my brother had a science experiment in which he had to buy a big thing of twine for it (this is why I asked TerryJ in the other post, how she knawed). He had a lot left over and one day when I was at the computer first he wanted it. He picked me up and carried me off the chair (I was 15, he was 17, obviously a female vs male, he was much stronger). He took a part of the twine and tied my hands behind my back. As I am screaming at him, he then cuts more twine, leads me back to the chair, makes me sit, and actually ties me to the chair). He left for a minute, all the while trying to stand up and move and yelling at him, he comes back with a feather duster and starts tickling me all the while screaming to stop. He got tired of my laughing so grabbed a bandana and tied it into my mouth. He let me go after about 20 minutes, and told me if I ever told he would do it again.
So I am not sure how I would have handled him, it is wrong to do that and people should say no and it should stop, but sometimes it happens without them listening.