OK, you can not remove "stingers" of jelly fish with anything, touching the nematocysts will cause them to eject the dart, they have a trigger mechanism with a "spring" like coil inside that shoots the stinger and venom out. There are thousands of them. What may look like one may be 50 or more.Quite a cool design. Use a credit card or something hard to scrape them off your skin but not guaranteed, fresh water will trigger them and eject instantly,Urine may work but not great Vinegar or ammonia will work OK but when the nematocysts ejects and the toxin is injected, no hope the venom is in there. The sting depends on the species of the jelly. Most will cause pain and a bit of swelling at the site for a couple of days, if someone is unfortunate enough to be stung by a Australian box jelly, they can and have killed people. Some of these jelly fish can sting through a thin wet suit and have a way of finding the parts you don't have covered. Not too big of a concern, but they hurt and can cause some underlying health issues, especially if you may be allergic to bees. I was covered head to toe by a Man-o-War a few years back, have scars on my forearms, neck, shoulders and chest. Vinegar did not work very well and I'd be damned if I was going to have my partner piss on me, so I used salt water hose on the boat and my credit card method I learned from another biologist. Worked well. Had to go to hospital, bad day. Was sick for about a week and skin hurt for a month or so in certain spots.
Stingray barbs are another story, they can be removed, and if removed the site should be cleaned. Their is more risk of infection, serious infection with this wound. I know a guy who lost his leg due to gangrene from a sting. It is purely a defensive sting and they do not go after you. Shuffle feet. I hope this helps.
Eric