My wife left a shop having paid for an item of clothing, put the clothing in a chest of drawers, threw away the receipt, and several weeks later came to wear said item, only to find the shop security tag still attached. What do you do? Well, we ummed and ahhhed over this for a while and thought that we'd probably be sent to a Turkish jail if we took it back to the shop (being the kind of "sports" shop it was), so I decided to try and get it off. Fortunately it wasn't the type that has a dye in it that wrecks your clothes, just a pin that's held in place by magnets.
I tried all sorts of magnets to try and release their grip - to no avail - and I've got some really big ones. I guess they've either got to be the right size, strength or shape - or possibly concentrate their magnetic field in a certain direction? I dunno. So, I took out my trusty large flat blade screwdriver, gently inserted it between the pin/cap assembly and the clothes item and started prising it up. Very slowly a gap started to appear. When the gap got big enough, I inserted some other material to protect the clothes and then really went for it with a pair of pliers and the screwdriver. Part of the cap broke in the process, but that didn't really bother me. The pin was coming out, and that was what was important. It was quite a struggle, but it eventually came free, leaving an undamaged pair of shorts. Result!
Anyway, I know there are loads of different types, so don't ask me what type this one is, but here's some pictures. Hope it helps someone [honest] somewhere: